Chapter 1: Introduction
1.1 What Is NASM?
The Netwide Assembler, NASM, is an 80x86 assembler designed for portability and
modularity. It supports a range of object file formats, including Linux a.out and ELF,
NetBSD/FreeBSD, COFF, Microsoft 16-bit OBJ and Win32. It will also output plain binary
files. Its syntax is designed to be simple and easy to understand, similar to Intel's but less
complex. It supports Pentium, P6 and MMX opcodes, and has macro capability.
1.1.1 Why Yet Another Assembler?
The Netwide Assembler grew out of an idea on comp.lang.asm.x86 (or possibly
alt.lang.asm - I forget which), which was essentially that there didn't seem to be a good
free x86-series assembler around, and that maybe someone ought to write one.

a86 is good, but not free, and in particular you don't get any 32-bit capability until you pay. It's DOS
only, too.

gas is free, and ports over DOS and Unix, but it's not very good, since it's designed to be a back end
to gcc, which always feeds it correct code. So its error checking is minimal. Also, its syntax is
horrible, from the point of view of anyone trying to actually write anything in it. Plus you can't write
16-bit code in it (properly).

as86 is Linux-specific, and (my version at least) doesn't seem to have much (or any)
documentation.

MASM isn't very good, and it's expensive, and it runs only under DOS.

TASM is better, but still strives for MASM compatibility, which means millions of directives and
tons of red tape. And its syntax is essentially MASM's, with the contradictions and quirks that entails
(although it sorts out some of those by means of Ideal mode). It's expensive too. And it's DOS-only.
So here, for your coding pleasure, is NASM. At present it's still in prototype stage - we
don't promise that it can outperform any of these assemblers. But please, please send us bug
reports, fixes, helpful information, and anything else you can get your hands on (and thanks
to the many people who've done this already! You all know who you are), and we'll
improve it out of all recognition. Again.
1.1.2 Licence Conditions
Please see the file Licence, supplied as part of any NASM distribution archive, for the
licence conditions under which you may use NASM.
1.2 Contact Information
The current version of NASM (since 0.98) are maintained by H. Peter Anvin,
hpa@zytor.com. If you want to report a bug, please read section 10.2 first.
NASM has a WWW page at http://www.cryogen.com/Nasm.
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The original authors are e-mailable as jules@earthcorp.com and anakin@pobox.com.
New releases of NASM are uploaded to ftp.kernel.org, sunsite.unc.edu,
ftp.simtel.net and ftp.coast.net. Announcements are posted to comp.lang.asm.x86,
alt.lang.asm, comp.os.linux.announce and comp.archives.msdos.announce (the
last one is done automagically by uploading to ftp.simtel.net).
If you don't have Usenet access, or would rather be informed by e-mail when new releases
come out, you can subscribe to the nasm-announce email list by sending an email
containing the line subscribe nasm-announce to majordomo@linux.kernel.org.
If you want information about NASM beta releases, please subscribe to the nasm-beta
email list by sending an email containing the line subscribe nasm-beta to
majordomo@linux.kernel.org.
1.3 Installation
1.3.1 Installing NASM under MS-DOS or Windows
Once you've obtained the DOS archive for NASM, nasmXXX.zip (where XXX denotes the
version number of NASM contained in the archive), unpack it into its own directory (for
example c:\nasm).
The archive will contain four executable files: the NASM executable files nasm.exe and
nasmw.exe, and the NDISASM executable files ndisasm.exe and ndisasmw.exe. In each
case, the file whose name ends in w is a Win32 executable, designed to run under Windows
95 or Windows NT Intel, and the other one is a 16-bit DOS executable.
The only file NASM needs to run is its own executable, so copy (at least) one of nasm.exe
and nasmw.exe to a directory on your PATH, or alternatively edit autoexec.bat to add the
nasm directory to your PATH. (If you're only installing the Win32 version, you may wish to
rename it to nasm.exe.)
That's it - NASM is installed. You don't need the nasm directory to be present to run NASM
(unless you've added it to your PATH), so you can delete it if you need to save space;
however, you may want to keep the documentation or test programs.
If you've downloaded the DOS source archive, nasmXXXs.zip, the nasm directory will also
contain the full NASM source code, and a selection of Makefiles you can (hopefully) use to
rebuild your copy of NASM from scratch. The file Readme lists the various Makefiles and
which compilers they work with.
Note that the source files insnsa.c, insnsd.c, insnsi.h and insnsn.c are automatically
generated from the master instruction table insns.dat by a Perl script; the file macros.c is
generated from standard.mac by another Perl script. Although the NASM 0.98
distribution includes these generated files, you will need to rebuild them (and hence, will
need a Perl interpreter) if you change insns.dat, standard.mac or the documentation. It
is possible future source distributions may not include these files at all. Ports of Perl for a
variety of platforms, including DOS and Windows, are available from www.cpan.org.
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1.3.2 Installing NASM under Unix
Once you've obtained the Unix source archive for NASM, nasm-X.XX.tar.gz (where X.XX
denotes the version number of NASM contained in the archive), unpack it into a directory
such as /usr/local/src. The archive, when unpacked, will create its own subdirectory
nasm-X.XX.
NASM is an auto-configuring package: once you've unpacked it, cd to the directory it's
been unpacked into and type ./configure. This shell script will find the best C compiler to
use for building NASM and set up Makefiles accordingly.
Once NASM has auto-configured, you can type make to build the nasm and ndisasm
binaries, and then make install to install them in /usr/local/bin and install the man
pages nasm.1 and ndisasm.1 in /usr/local/man/man1. Alternatively, you can give
options such as --prefix to the configure script (see the file INSTALL for more details),
or install the programs yourself.
NASM also comes with a set of utilities for handling the RDOFF custom object-file format,
which are in the rdoff subdirectory of the NASM archive. You can build these with make
rdf and install them with make rdf_install, if you want them.
If NASM fails to auto-configure, you may still be able to make it compile by using the fallback Unix makefile Makefile.unx. Copy or rename that file to Makefile and try typing
make. There is also a Makefile.unx file in the rdoff subdirectory.
Chapter 2: Running NASM
2.1 NASM Command-Line Syntax
To assemble a file, you issue a command of the form
nasm -f <format> <filename> [-o <output>]
For example,
nasm -f elf myfile.asm
will assemble myfile.asm into an ELF object file myfile.o. And
nasm -f bin myfile.asm -o myfile.com
will assemble myfile.asm into a raw binary file myfile.com.
To produce a listing file, with the hex codes output from NASM displayed on the left of the
original sources, use the -l option to give a listing file name, for example:
nasm -f coff myfile.asm -l myfile.lst
To get further usage instructions from NASM, try typing
nasm -h
This will also list the available output file formats, and what they are.
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If you use Linux but aren't sure whether your system is a.out or ELF, type
file nasm
(in the directory in which you put the NASM binary when you installed it). If it says
something like
nasm: ELF 32-bit LSB executable i386 (386 and up) Version 1
then your system is ELF, and you should use the option -f elf when you want NASM to
produce Linux object files. If it says
nasm: Linux/i386 demand-paged executable (QMAGIC)
or something similar, your system is a.out, and you should use -f aout instead (Linux
a.out systems are considered obsolete, and are rare these days.)
Like Unix compilers and assemblers, NASM is silent unless it goes wrong: you won't see
any output at all, unless it gives error messages.
2.1.1 The -o Option: Specifying the Output File Name
NASM will normally choose the name of your output file for you; precisely how it does
this is dependent on the object file format. For Microsoft object file formats (obj and
win32), it will remove the .asm extension (or whatever extension you like to use - NASM
doesn't care) from your source file name and substitute .obj. For Unix object file formats
(aout, coff, elf and as86) it will substitute .o. For rdf, it will use .rdf, and for the bin
format it will simply remove the extension, so that myfile.asm produces the output file
myfile.
If the output file already exists, NASM will overwrite it, unless it has the same name as the
input file, in which case it will give a warning and use nasm.out as the output file name
instead.
For situations in which this behaviour is unacceptable, NASM provides the -o commandline option, which allows you to specify your desired output file name. You invoke -o by
following it with the name you wish for the output file, either with or without an
intervening space. For example:
nasm -f bin program.asm -o program.com
nasm -f bin driver.asm -odriver.sys
2.1.2 The -f Option: Specifying the Output File Format
If you do not supply the -f option to NASM, it will choose an output file format for you
itself. In the distribution versions of NASM, the default is always bin; if you've compiled
your own copy of NASM, you can redefine OF_DEFAULT at compile time and choose what
you want the default to be.
Like -o, the intervening space between -f and the output file format is optional; so -f elf
and -felf are both valid.
A complete list of the available output file formats can be given by issuing the command
nasm -h.
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2.1.3 The -l Option: Generating a Listing File
If you supply the -l option to NASM, followed (with the usual optional space) by a file
name, NASM will generate a source-listing file for you, in which addresses and generated
code are listed on the left, and the actual source code, with expansions of multi-line macros
(except those which specifically request no expansion in source listings: see section 4.2.9)
on the right. For example:
nasm -f elf myfile.asm -l myfile.lst
2.1.4 The -E Option: Send Errors to a File
Under MS-DOS it can be difficult (though there are ways) to redirect the standard-error
output of a program to a file. Since NASM usually produces its warning and error messages
on stderr, this can make it hard to capture the errors if (for example) you want to load
them into an editor.
NASM therefore provides the -E option, taking a filename argument which causes errors to
be sent to the specified files rather than standard error. Therefore you can redirect the errors
into a file by typing
nasm -E myfile.err -f obj myfile.asm
2.1.5 The -s Option: Send Errors to stdout
The -s option redirects error messages to stdout rather than stderr, so it can be
redirected under MS-DOS. To assemble the file myfile.asm and pipe its output to the more
program, you can type:
nasm -s -f obj myfile.asm | more
See also the -E option, section 2.1.4.
2.1.6 The -i Option: Include File Search Directories
When NASM sees the %include directive in a source file (see section 4.5), it will search
for the given file not only in the current directory, but also in any directories specified on
the command line by the use of the -i option. Therefore you can include files from a macro
library, for example, by typing
nasm -ic:\macrolib\ -f obj myfile.asm
(As usual, a space between -i and the path name is allowed, and optional).
NASM, in the interests of complete source-code portability, does not understand the file
naming conventions of the OS it is running on; the string you provide as an argument to the
-i option will be prepended exactly as written to the name of the include file. Therefore the
trailing backslash in the above example is necessary. Under Unix, a trailing forward slash is
similarly necessary.
(You can use this to your advantage, if you're really perverse, by noting that the option ifoo will cause %include "bar.i" to search for the file foobar.i...)
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If you want to define a standard include search path, similar to /usr/include on Unix
systems, you should place one or more -i directives in the NASM environment variable (see
section 2.1.13).
For Makefile compatibility with many C compilers, this option can also be specified as -I.
2.1.7 The -p Option: Pre-Include a File
NASM allows you to specify files to be pre-included into your source file, by the use of the
-p option. So running
nasm myfile.asm -p myinc.inc
is equivalent to running nasm myfile.asm and placing the directive %include
"myinc.inc" at the start of the file.
For consistency with the -I, -D and -U options, this option can also be specified as -P.
2.1.8 The -d Option: Pre-Define a Macro
Just as the -p option gives an alternative to placing %include directives at the start of a
source file, the -d option gives an alternative to placing a %define directive. You could
code
nasm myfile.asm -dFOO=100
as an alternative to placing the directive
%define FOO 100
at the start of the file. You can miss off the macro value, as well: the option -dFOO is
equivalent to coding %define FOO. This form of the directive may be useful for selecting
assembly-time options which are then tested using %ifdef, for example -dDEBUG.
For Makefile compatibility with many C compilers, this option can also be specified as -D.
2.1.9 The -u Option: Undefine a Macro
The -u option undefines a macro that would otherwise have been pre-defined, either
automatically or by a -p or -d option specified earlier on the command lines.
For example, the following command line:
nasm myfile.asm -dFOO=100 -uFOO
would result in FOO not being a predefined macro in the program. This is useful to override
options specified at a different point in a Makefile.
For Makefile compatibility with many C compilers, this option can also be specified as -U.
2.1.10 The -e Option: Preprocess Only
NASM allows the preprocessor to be run on its own, up to a point. Using the -e option
(which requires no arguments) will cause NASM to preprocess its input file, expand all the
6
macro references, remove all the comments and preprocessor directives, and print the
resulting file on standard output (or save it to a file, if the -o option is also used).
This option cannot be applied to programs which require the preprocessor to evaluate
expressions which depend on the values of symbols: so code such as
%assign tablesize ($-tablestart)
will cause an error in preprocess-only mode.
2.1.11 The -a Option: Don't Preprocess At All
If NASM is being used as the back end to a compiler, it might be desirable to suppress
preprocessing completely and assume the compiler has already done it, to save time and
increase compilation speeds. The -a option, requiring no argument, instructs NASM to
replace its powerful preprocessor with a stub preprocessor which does nothing.
2.1.12 The -w Option: Enable or Disable Assembly Warnings
NASM can observe many conditions during the course of assembly which are worth
mentioning to the user, but not a sufficiently severe error to justify NASM refusing to
generate an output file. These conditions are reported like errors, but come up with the
word `warning' before the message. Warnings do not prevent NASM from generating an
output file and returning a success status to the operating system.
Some conditions are even less severe than that: they are only sometimes worth mentioning
to the user. Therefore NASM supports the -w command-line option, which enables or
disables certain classes of assembly warning. Such warning classes are described by a
name, for example orphan-labels; you can enable warnings of this class by the
command-line option -w+orphan-labels and disable it by -w-orphan-labels.
The suppressible warning classes are:

macro-params covers warnings about multi-line macros being invoked with the wrong number of
parameters. This warning class is enabled by default; see section 4.2.1 for an example of why you
might want to disable it.

orphan-labels covers warnings about source lines which contain no instruction but define a
label without a trailing colon. NASM does not warn about this somewhat obscure condition by
default; see section 3.1 for an example of why you might want it to.

number-overflow covers warnings about numeric constants which don't fit in 32 bits (for
example, it's easy to type one too many Fs and produce 0x7ffffffff by mistake). This warning
class is enabled by default.
2.1.13 The NASM Environment Variable
If you define an environment variable called NASM, the program will interpret it as a list of
extra command-line options, which are processed before the real command line. You can
use this to define standard search directories for include files, by putting -i options in the
NASM variable.
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The value of the variable is split up at white space, so that the value -s -ic:\nasmlib will
be treated as two separate options. However, that means that the value -dNAME="my name"
won't do what you might want, because it will be split at the space and the NASM
command-line processing will get confused by the two nonsensical words -dNAME="my and
name".
To get round this, NASM provides a feature whereby, if you begin the NASM environment
variable with some character that isn't a minus sign, then NASM will treat this character as
the separator character for options. So setting the NASM variable to the value !-s!ic:\nasmlib is equivalent to setting it to -s -ic:\nasmlib, but !-dNAME="my name" will
work.
2.2 Quick Start for MASM Users
If you're used to writing programs with MASM, or with TASM in MASM-compatible
(non-Ideal) mode, or with a86, this section attempts to outline the major differences
between MASM's syntax and NASM's. If you're not already used to MASM, it's probably
worth skipping this section.
2.2.1 NASM Is Case-Sensitive
One simple difference is that NASM is case-sensitive. It makes a difference whether you
call your label foo, Foo or FOO. If you're assembling to DOS or OS/2 .OBJ files, you can
invoke the UPPERCASE directive (documented in section 6.2) to ensure that all symbols
exported to other code modules are forced to be upper case; but even then, within a single
module, NASM will distinguish between labels differing only in case.
2.2.2 NASM Requires Square Brackets For Memory References
NASM was designed with simplicity of syntax in mind. One of the design goals of NASM
is that it should be possible, as far as is practical, for the user to look at a single line of
NASM code and tell what opcode is generated by it. You can't do this in MASM: if you
declare, for example,
foo
bar
equ 1
dw 2
then the two lines of code
mov ax,foo
mov ax,bar
generate completely different opcodes, despite having identical-looking syntaxes.
NASM avoids this undesirable situation by having a much simpler syntax for memory
references. The rule is simply that any access to the contents of a memory location requires
square brackets around the address, and any access to the address of a variable doesn't. So
an instruction of the form mov ax,foo will always refer to a compile-time constant,
whether it's an EQU or the address of a variable; and to access the contents of the variable
bar, you must code mov ax,[bar].
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This also means that NASM has no need for MASM's OFFSET keyword, since the MASM
code mov ax,offset bar means exactly the same thing as NASM's mov ax,bar. If you're
trying to get large amounts of MASM code to assemble sensibly under NASM, you can
always code %idefine offset to make the preprocessor treat the OFFSET keyword as a noop.
This issue is even more confusing in a86, where declaring a label with a trailing colon
defines it to be a `label' as opposed to a `variable' and causes a86 to adopt NASM-style
semantics; so in a86, mov ax,var has different behaviour depending on whether var was
declared as var: dw 0 (a label) or var dw 0 (a word-size variable). NASM is very simple
by comparison: everything is a label.
NASM, in the interests of simplicity, also does not support the hybrid syntaxes supported
by MASM and its clones, such as mov ax,table[bx], where a memory reference is
denoted by one portion outside square brackets and another portion inside. The correct
syntax for the above is mov ax,[table+bx]. Likewise, mov ax,es:[di] is wrong and mov
ax,[es:di] is right.
2.2.3 NASM Doesn't Store Variable Types
NASM, by design, chooses not to remember the types of variables you declare. Whereas
MASM will remember, on seeing var dw 0, that you declared var as a word-size variable,
and will then be able to fill in the ambiguity in the size of the instruction mov var,2,
NASM will deliberately remember nothing about the symbol var except where it begins,
and so you must explicitly code mov word [var],2.
For this reason, NASM doesn't support the LODS, MOVS, STOS, SCAS, CMPS, INS, or OUTS
instructions, but only supports the forms such as LODSB, MOVSW, and SCASD, which explicitly
specify the size of the components of the strings being manipulated.
2.2.4 NASM Doesn't ASSUME
As part of NASM's drive for simplicity, it also does not support the ASSUME directive.
NASM will not keep track of what values you choose to put in your segment registers, and
will never automatically generate a segment override prefix.
2.2.5 NASM Doesn't Support Memory Models
NASM also does not have any directives to support different 16-bit memory models. The
programmer has to keep track of which functions are supposed to be called with a far call
and which with a near call, and is responsible for putting the correct form of RET instruction
(RETN or RETF; NASM accepts RET itself as an alternate form for RETN); in addition, the
programmer is responsible for coding CALL FAR instructions where necessary when
calling external functions, and must also keep track of which external variable definitions
are far and which are near.
2.2.6 Floating-Point Differences
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NASM uses different names to refer to floating-point registers from MASM: where MASM
would call them ST(0), ST(1) and so on, and a86 would call them simply 0, 1 and so on,
NASM chooses to call them st0, st1 etc.
As of version 0.96, NASM now treats the instructions with `nowait' forms in the same way
as MASM-compatible assemblers. The idiosyncratic treatment employed by 0.95 and
earlier was based on a misunderstanding by the authors.
2.2.7 Other Differences
For historical reasons, NASM uses the keyword TWORD where MASM and compatible
assemblers use TBYTE.
NASM does not declare uninitialised storage in the same way as MASM: where a MASM
programmer might use stack db 64 dup (?), NASM requires stack resb 64, intended
to be read as `reserve 64 bytes'. For a limited amount of compatibility, since NASM treats ?
as a valid character in symbol names, you can code ? equ 0 and then writing dw ? will at
least do something vaguely useful. DUP is still not a supported syntax, however.
In addition to all of this, macros and directives work completely differently to MASM. See
chapter 4 and chapter 5 for further details.
Chapter 3: The NASM Language
3.1 Layout of a NASM Source Line
Like most assemblers, each NASM source line contains (unless it is a macro, a
preprocessor directive or an assembler directive: see chapter 4 and chapter 5) some
combination of the four fields
label:
instruction operands
; comment
As usual, most of these fields are optional; the presence or absence of any combination of a
label, an instruction and a comment is allowed. Of course, the operand field is either
required or forbidden by the presence and nature of the instruction field.
NASM places no restrictions on white space within a line: labels may have white space
before them, or instructions may have no space before them, or anything. The colon after a
label is also optional. (Note that this means that if you intend to code lodsb alone on a line,
and type lodab by accident, then that's still a valid source line which does nothing but
define a label. Running NASM with the command-line option -w+orphan-labels will
cause it to warn you if you define a label alone on a line without a trailing colon.)
Valid characters in labels are letters, numbers, _, $, #, @, ~, ., and ?. The only characters
which may be used as the first character of an identifier are letters, . (with special meaning:
see section 3.8), _ and ?. An identifier may also be prefixed with a $ to indicate that it is
intended to be read as an identifier and not a reserved word; thus, if some other module you
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are linking with defines a symbol called eax, you can refer to $eax in NASM code to
distinguish the symbol from the register.
The instruction field may contain any machine instruction: Pentium and P6 instructions,
FPU instructions, MMX instructions and even undocumented instructions are all supported.
The instruction may be prefixed by LOCK, REP, REPE/REPZ or REPNE/REPNZ, in the usual
way. Explicit address-size and operand-size prefixes A16, A32, O16 and O32 are provided one example of their use is given in chapter 9. You can also use the name of a segment
register as an instruction prefix: coding es mov [bx],ax is equivalent to coding mov
[es:bx],ax. We recommend the latter syntax, since it is consistent with other syntactic
features of the language, but for instructions such as LODSB, which has no operands and yet
can require a segment override, there is no clean syntactic way to proceed apart from es
lodsb.
An instruction is not required to use a prefix: prefixes such as CS, A32, LOCK or REPE can
appear on a line by themselves, and NASM will just generate the prefix bytes.
In addition to actual machine instructions, NASM also supports a number of pseudoinstructions, described in section 3.2.
Instruction operands may take a number of forms: they can be registers, described simply
by the register name (e.g. ax, bp, ebx, cr0: NASM does not use the gas-style syntax in
which register names must be prefixed by a % sign), or they can be effective addresses (see
section 3.3), constants (section 3.4) or expressions (section 3.5).
For floating-point instructions, NASM accepts a wide range of syntaxes: you can use twooperand forms like MASM supports, or you can use NASM's native single-operand forms
in most cases. Details of all forms of each supported instruction are given in appendix A.
For example, you can code:
fadd st1
fadd st0,st1
; this sets st0 := st0 + st1
; so does this
fadd st1,st0
fadd to st1
; this sets st1 := st1 + st0
; so does this
Almost any floating-point instruction that references memory must use one of the prefixes
DWORD, QWORD or TWORD to indicate what size of memory operand it refers to.
3.2 Pseudo-Instructions
Pseudo-instructions are things which, though not real x86 machine instructions, are used in
the instruction field anyway because that's the most convenient place to put them. The
current pseudo-instructions are DB, DW, DD, DQ and DT, their uninitialised counterparts RESB,
RESW, RESD, RESQ and REST, the INCBIN command, the EQU command, and the TIMES prefix.
3.2.1 DB and friends: Declaring Initialised Data
DB, DW, DD, DQ
and DT are used, much as in MASM, to declare initialised data in the output
file. They can be invoked in a wide range of ways:
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db
db
db
db
dw
dw
dw
dw
dd
dd
dq
dt
DQ
0x55
0x55,0x56,0x57
'a',0x55
'hello',13,10,'$'
0x1234
'a'
'ab'
'abc'
0x12345678
1.234567e20
1.234567e20
1.234567e20
;
;
;
;
;
;
;
;
;
;
;
;
just the byte 0x55
three bytes in succession
character constants are OK
so are string constants
0x34 0x12
0x41 0x00 (it's just a number)
0x41 0x42 (character constant)
0x41 0x42 0x43 0x00 (string)
0x78 0x56 0x34 0x12
floating-point constant
double-precision float
extended-precision float
and DT do not accept numeric constants or string constants as operands.
3.2.2 RESB and friends: Declaring Uninitialised Data
RESB, RESW, RESD, RESQ
and REST are designed to be used in the BSS section of a module:
they declare uninitialised storage space. Each takes a single operand, which is the number
of bytes, words, doublewords or whatever to reserve. As stated in section 2.2.7, NASM
does not support the MASM/TASM syntax of reserving uninitialised space by writing DW ?
or similar things: this is what it does instead. The operand to a RESB-type pseudoinstruction is a critical expression: see section 3.7.
For example:
buffer:
resb 64
wordvar: resw 1
realarray resq 10
; reserve 64 bytes
; reserve a word
; array of ten reals
3.2.3 INCBIN: Including External Binary Files
INCBIN
is borrowed from the old Amiga assembler DevPac: it includes a binary file
verbatim into the output file. This can be handy for (for example) including graphics and
sound data directly into a game executable file. It can be called in one of these three ways:
incbin "file.dat"
; include the whole file
incbin "file.dat",1024 ; skip the first 1024 bytes
incbin "file.dat",1024,512 ; skip the first 1024, and
; actually include at most 512
3.2.4 EQU: Defining Constants
EQU
defines a symbol to a given constant value: when EQU is used, the source line must
contain a label. The action of EQU is to define the given label name to the value of its (only)
operand. This definition is absolute, and cannot change later. So, for example,
message
msglen
db 'hello, world'
equ $-message
defines msglen to be the constant 12. msglen may not then be redefined later. This is not a
preprocessor definition either: the value of msglen is evaluated once, using the value of $
(see section 3.5 for an explanation of $) at the point of definition, rather than being
evaluated wherever it is referenced and using the value of $ at the point of reference. Note
that the operand to an EQU is also a critical expression (section 3.7).
12
3.2.5 TIMES: Repeating Instructions or Data
The TIMES prefix causes the instruction to be assembled multiple times. This is partly
present as NASM's equivalent of the DUP syntax supported by MASM-compatible
assemblers, in that you can code
zerobuf:
times 64 db 0
or similar things; but TIMES is more versatile than that. The argument to TIMES is not just a
numeric constant, but a numeric expression, so you can do things like
buffer:
db 'hello, world'
times 64-$+buffer db ' '
which will store exactly enough spaces to make the total length of buffer up to 64. Finally,
TIMES can be applied to ordinary instructions, so you can code trivial unrolled loops in it:
times 100 movsb
Note that there is no effective difference between times 100 resb 1 and resb 100,
except that the latter will be assembled about 100 times faster due to the internal structure
of the assembler.
The operand to TIMES, like that of EQU and those of RESB and friends, is a critical
expression (section 3.7).
Note also that TIMES can't be applied to macros: the reason for this is that TIMES is
processed after the macro phase, which allows the argument to TIMES to contain
expressions such as 64-$+buffer as above. To repeat more than one line of code, or a
complex macro, use the preprocessor %rep directive.
3.3 Effective Addresses
An effective address is any operand to an instruction which references memory. Effective
addresses, in NASM, have a very simple syntax: they consist of an expression evaluating to
the desired address, enclosed in square brackets. For example:
wordvar
dw 123
mov ax,[wordvar]
mov ax,[wordvar+1]
mov ax,[es:wordvar+bx]
Anything not conforming to this simple system is not a valid memory reference in NASM,
for example es:wordvar[bx].
More complicated effective addresses, such as those involving more than one register, work
in exactly the same way:
mov eax,[ebx*2+ecx+offset]
mov ax,[bp+di+8]
NASM is capable of doing algebra on these effective addresses, so that things which don't
necessarily look legal are perfectly all right:
mov eax,[ebx*5]
; assembles as [ebx*4+ebx]
mov eax,[label1*2-label2] ; ie [label1+(label1-label2)]
13
Some forms of effective address have more than one assembled form; in most such cases
NASM will generate the smallest form it can. For example, there are distinct assembled
forms for the 32-bit effective addresses [eax*2+0] and [eax+eax], and NASM will
generally generate the latter on the grounds that the former requires four bytes to store a
zero offset.
NASM has a hinting mechanism which will cause [eax+ebx] and [ebx+eax] to generate
different opcodes; this is occasionally useful because [esi+ebp] and [ebp+esi] have
different default segment registers.
However, you can force NASM to generate an effective address in a particular form by the
use of the keywords BYTE, WORD, DWORD and NOSPLIT. If you need [eax+3] to be assembled
using a double-word offset field instead of the one byte NASM will normally generate, you
can code [dword eax+3]. Similarly, you can force NASM to use a byte offset for a small
value which it hasn't seen on the first pass (see section 3.7 for an example of such a code
fragment) by using [byte eax+offset]. As special cases, [byte eax] will code [eax+0]
with a byte offset of zero, and [dword eax] will code it with a double-word offset of zero.
The normal form, [eax], will be coded with no offset field.
Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to
be absent and space to be saved; in fact, it will also split [eax*2+offset] into
[eax+eax+offset]. You can combat this behaviour by the use of the NOSPLIT keyword:
[nosplit eax*2] will force [eax*2+0] to be generated literally.
3.4 Constants
NASM understands four different types of constant: numeric, character, string and floatingpoint.
3.4.1 Numeric Constants
A numeric constant is simply a number. NASM allows you to specify numbers in a variety
of number bases, in a variety of ways: you can suffix H, Q and B for hex, octal and binary, or
you can prefix 0x for hex in the style of C, or you can prefix $ for hex in the style of
Borland Pascal. Note, though, that the $ prefix does double duty as a prefix on identifiers
(see section 3.1), so a hex number prefixed with a $ sign must have a digit after the $ rather
than a letter.
Some examples:
mov
mov
mov
mov
mov
mov
ax,100
ax,0a2h
ax,$0a2
ax,0xa2
ax,777q
ax,10010011b
;
;
;
;
;
;
decimal
hex
hex again: the 0 is required
hex yet again
octal
binary
3.4.2 Character Constants
14
A character constant consists of up to four characters enclosed in either single or double
quotes. The type of quote makes no difference to NASM, except of course that surrounding
the constant with single quotes allows double quotes to appear within it and vice versa.
A character constant with more than one character will be arranged with little-endian order
in mind: if you code
mov eax,'abcd'
then the constant generated is not 0x61626364, but 0x64636261, so that if you were then to
store the value into memory, it would read abcd rather than dcba. This is also the sense of
character constants understood by the Pentium's CPUID instruction (see section A.22).
3.4.3 String Constants
String constants are only acceptable to some pseudo-instructions, namely the DB family and
INCBIN.
A string constant looks like a character constant, only longer. It is treated as a
concatenation of maximum-size character constants for the conditions. So the following are
equivalent:
db 'hello'
; string constant
db 'h','e','l','l','o' ; equivalent character constants
And the following are also equivalent:
dd 'ninechars'
dd 'nine','char','s'
db 'ninechars',0,0,0
; doubleword string constant
; becomes three doublewords
; and really looks like this
Note that when used as an operand to db, a constant like 'ab' is treated as a string constant
despite being short enough to be a character constant, because otherwise db 'ab' would
have the same effect as db 'a', which would be silly. Similarly, three-character or fourcharacter constants are treated as strings when they are operands to dw.
3.4.4 Floating-Point Constants
Floating-point constants are acceptable only as arguments to DD, DQ and DT. They are
expressed in the traditional form: digits, then a period, then optionally more digits, then
optionally an E followed by an exponent. The period is mandatory, so that NASM can
distinguish between dd 1, which declares an integer constant, and dd 1.0 which declares a
floating-point constant.
Some examples:
dd
dq
dq
dq
dt
1.2
; an easy one
1.e10
; 10,000,000,000
1.e+10
; synonymous with 1.e10
1.e-10
; 0.000 000 000 1
3.141592653589793238462 ; pi
NASM cannot do compile-time arithmetic on floating-point constants. This is because
NASM is designed to be portable - although it always generates code to run on x86
processors, the assembler itself can run on any system with an ANSI C compiler.
15
Therefore, the assembler cannot guarantee the presence of a floating-point unit capable of
handling the Intel number formats, and so for NASM to be able to do floating arithmetic it
would have to include its own complete set of floating-point routines, which would
significantly increase the size of the assembler for very little benefit.
3.5 Expressions
Expressions in NASM are similar in syntax to those in C.
NASM does not guarantee the size of the integers used to evaluate expressions at compile
time: since NASM can compile and run on 64-bit systems quite happily, don't assume that
expressions are evaluated in 32-bit registers and so try to make deliberate use of integer
overflow. It might not always work. The only thing NASM will guarantee is what's
guaranteed by ANSI C: you always have at least 32 bits to work in.
NASM supports two special tokens in expressions, allowing calculations to involve the
current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the
beginning of the line containing the expression; so you can code an infinite loop using JMP
$. $$ evaluates to the beginning of the current section; so you can tell how far into the
section you are by using ($-$$).
The arithmetic operators provided by NASM are listed here, in increasing order of
precedence.
3.5.1 |: Bitwise OR Operator
The | operator gives a bitwise OR, exactly as performed by the OR machine instruction.
Bitwise OR is the lowest-priority arithmetic operator supported by NASM.
3.5.2 ^: Bitwise XOR Operator
^
provides the bitwise XOR operation.
3.5.3 &: Bitwise AND Operator
&
provides the bitwise AND operation.
3.5.4 << and >>: Bit Shift Operators
<<
gives a bit-shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >>
gives a bit-shift to the right; in NASM, such a shift is always unsigned, so that the bits
shifted in from the left-hand end are filled with zero rather than a sign-extension of the
previous highest bit.
3.5.5 + and -: Addition and Subtraction Operators
The + and - operators do perfectly ordinary addition and subtraction.
3.5.6 *, /, //, % and %%: Multiplication and Division
16
*
is the multiplication operator. / and // are both division operators: / is unsigned division
and // is signed division. Similarly, % and %% provide unsigned and signed modulo
operators respectively.
NASM, like ANSI C, provides no guarantees about the sensible operation of the signed
modulo operator.
Since the % character is used extensively by the macro preprocessor, you should ensure that
both the signed and unsigned modulo operators are followed by white space wherever they
appear.
3.5.7 Unary Operators: +, -, ~ and SEG
The highest-priority operators in NASM's expression grammar are those which only apply
to one argument. - negates its operand, + does nothing (it's provided for symmetry with -),
~ computes the one's complement of its operand, and SEG provides the segment address of
its operand (explained in more detail in section 3.6).
3.6 SEG and WRT
When writing large 16-bit programs, which must be split into multiple segments, it is often
necessary to be able to refer to the segment part of the address of a symbol. NASM
supports the SEG operator to perform this function.
The SEG operator returns the preferred segment base of a symbol, defined as the segment
base relative to which the offset of the symbol makes sense. So the code
mov ax,seg symbol
mov es,ax
mov bx,symbol
will load ES:BX with a valid pointer to the symbol symbol.
Things can be more complex than this: since 16-bit segments and groups may overlap, you
might occasionally want to refer to some symbol using a different segment base from the
preferred one. NASM lets you do this, by the use of the WRT (With Reference To) keyword.
So you can do things like
mov ax,weird_seg
; weird_seg is a segment base
mov es,ax
mov bx,symbol wrt weird_seg
to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol.
NASM supports far (inter-segment) calls and jumps by means of the syntax call
segment:offset, where segment and offset both represent immediate values. So to call a
far procedure, you could code either of
call (seg procedure):procedure
call weird_seg:(procedure wrt weird_seg)
(The parentheses are included for clarity, to show the intended parsing of the above
instructions. They are not necessary in practice.)
17
NASM supports the syntax call far procedure as a synonym for the first of the above
usages. JMP works identically to CALL in these examples.
To declare a far pointer to a data item in a data segment, you must code
dw symbol, seg symbol
NASM supports no convenient synonym for this, though you can always invent one using
the macro processor.
3.7 Critical Expressions
A limitation of NASM is that it is a two-pass assembler; unlike TASM and others, it will
always do exactly two assembly passes. Therefore it is unable to cope with source files that
are complex enough to require three or more passes.
The first pass is used to determine the size of all the assembled code and data, so that the
second pass, when generating all the code, knows all the symbol addresses the code refers
to. So one thing NASM can't handle is code whose size depends on the value of a symbol
declared after the code in question. For example,
label:
times (label-$) db 0
db 'Where am I?'
The argument to TIMES in this case could equally legally evaluate to anything at all; NASM
will reject this example because it cannot tell the size of the TIMES line when it first sees it.
It will just as firmly reject the slightly paradoxical code
label:
times (label-$+1) db 0
db 'NOW where am I?'
in which any value for the TIMES argument is by definition wrong!
NASM rejects these examples by means of a concept called a critical expression, which is
defined to be an expression whose value is required to be computable in the first pass, and
which must therefore depend only on symbols defined before it. The argument to the TIMES
prefix is a critical expression; for the same reason, the arguments to the RESB family of
pseudo-instructions are also critical expressions.
Critical expressions can crop up in other contexts as well: consider the following code.
symbol1
symbol2:
mov ax,symbol1
equ symbol2
On the first pass, NASM cannot determine the value of symbol1, because symbol1 is
defined to be equal to symbol2 which NASM hasn't seen yet. On the second pass,
therefore, when it encounters the line mov ax,symbol1, it is unable to generate the code for
it because it still doesn't know the value of symbol1. On the next line, it would see the EQU
again and be able to determine the value of symbol1, but by then it would be too late.
NASM avoids this problem by defining the right-hand side of an EQU statement to be a
critical expression, so the definition of symbol1 would be rejected in the first pass.
There is a related issue involving forward references: consider this code fragment.
18
offset
mov eax,[ebx+offset]
equ 10
NASM, on pass one, must calculate the size of the instruction mov eax,[ebx+offset]
without knowing the value of offset. It has no way of knowing that offset is small
enough to fit into a one-byte offset field and that it could therefore get away with
generating a shorter form of the effective-address encoding; for all it knows, in pass one,
offset could be a symbol in the code segment, and it might need the full four-byte form.
So it is forced to compute the size of the instruction to accommodate a four-byte address
part. In pass two, having made this decision, it is now forced to honour it and keep the
instruction large, so the code generated in this case is not as small as it could have been.
This problem can be solved by defining offset before using it, or by forcing byte size in
the effective address by coding [byte ebx+offset].
3.8 Local Labels
NASM gives special treatment to symbols beginning with a period. A label beginning with
a single period is treated as a local label, which means that it is associated with the previous
non-local label. So, for example:
label1
.loop
label2
.loop
; some code
; some more code
jne .loop
ret
; some code
; some more code
jne .loop
ret
In the above code fragment, each JNE instruction jumps to the line immediately before it,
because the two definitions of .loop are kept separate by virtue of each being associated
with the previous non-local label.
This form of local label handling is borrowed from the old Amiga assembler DevPac;
however, NASM goes one step further, in allowing access to local labels from other parts
of the code. This is achieved by means of defining a local label in terms of the previous
non-local label: the first definition of .loop above is really defining a symbol called
label1.loop, and the second defines a symbol called label2.loop. So, if you really
needed to, you could write
label3
; some more code
; and some more
jmp label1.loop
Sometimes it is useful - in a macro, for instance - to be able to define a label which can be
referenced from anywhere but which doesn't interfere with the normal local-label
mechanism. Such a label can't be non-local because it would interfere with subsequent
definitions of, and references to, local labels; and it can't be local because the macro that
defined it wouldn't know the label's full name. NASM therefore introduces a third type of
label, which is probably only useful in macro definitions: if a label begins with the special
prefix ..@, then it does nothing to the local label mechanism. So you could code
label1:
.local:
; a non-local label
; this is really label1.local
19
..@foo:
label2:
.local:
; this is a special symbol
; another non-local label
; this is really label2.local
jmp ..@foo
; this will jump three lines up
NASM has the capacity to define other special symbols beginning with a double period: for
example, ..start is used to specify the entry point in the obj output format (see section
6.2.6).
Chapter 4: The NASM Preprocessor
NASM contains a powerful macro processor, which supports conditional assembly, multilevel file inclusion, two forms of macro (single-line and multi-line), and a `context stack'
mechanism for extra macro power. Preprocessor directives all begin with a % sign.
4.1 Single-Line Macros
4.1.1 The Normal Way: %define
Single-line macros are defined using the %define preprocessor directive. The definitions
work in a similar way to C; so you can do things like
%define ctrl 0x1F &
%define param(a,b) ((a)+(a)*(b))
mov byte [param(2,ebx)], ctrl 'D'
which will expand to
mov byte [(2)+(2)*(ebx)], 0x1F & 'D'
When the expansion of a single-line macro contains tokens which invoke another macro,
the expansion is performed at invocation time, not at definition time. Thus the code
%define a(x) 1+b(x)
%define b(x) 2*x
mov ax,a(8)
will evaluate in the expected way to mov ax,1+2*8, even though the macro b wasn't
defined at the time of definition of a.
Macros defined with %define are case sensitive: after %define foo bar, only foo will
expand to bar: Foo or FOO will not. By using %idefine instead of %define (the `i' stands
for `insensitive') you can define all the case variants of a macro at once, so that %idefine
foo bar would cause foo, Foo, FOO, fOO and so on all to expand to bar.
There is a mechanism which detects when a macro call has occurred as a result of a
previous expansion of the same macro, to guard against circular references and infinite
loops. If this happens, the preprocessor will only expand the first occurrence of the macro.
Hence, if you code
%define a(x) 1+a(x)
mov ax,a(3)
20
the macro a(3) will expand once, becoming 1+a(3), and will then expand no further. This
behaviour can be useful: see section 8.1 for an example of its use.
You can overload single-line macros: if you write
%define foo(x) 1+x
%define foo(x,y) 1+x*y
the preprocessor will be able to handle both types of macro call, by counting the parameters
you pass; so foo(3) will become 1+3 whereas foo(ebx,2) will become 1+ebx*2.
However, if you define
%define foo bar
then no other definition of foo will be accepted: a macro with no parameters prohibits the
definition of the same name as a macro with parameters, and vice versa.
This doesn't prevent single-line macros being redefined: you can perfectly well define a
macro with
%define foo bar
and then re-define it later in the same source file with
%define foo baz
Then everywhere the macro foo is invoked, it will be expanded according to the most
recent definition. This is particularly useful when defining single-line macros with %assign
(see section 4.1.3).
You can pre-define single-line macros using the `-d' option on the NASM command line:
see section 2.1.8.
4.1.2 Undefining macros: %undef
Single-line macros can be removed with the %undef command. For example, the following
sequence:
%define foo bar
%undef foo
mov eax, foo
will expand to the instruction mov eax, foo, since after %undef the macro foo is no longer
defined.
Macros that would otherwise be pre-defined can be undefined on the command-line using
the `-u' option on the NASM command line: see section 2.1.9.
4.1.3 Preprocessor Variables: %assign
An alternative way to define single-line macros is by means of the %assign command (and
its case sensitivecase-insensitive counterpart %iassign, which differs from %assign in
exactly the same way that %idefine differs from %define).
%assign
is used to define single-line macros which take no parameters and have a numeric
value. This value can be specified in the form of an expression, and it will be evaluated
once, when the %assign directive is processed.
21
Like %define, macros defined using %assign can be re-defined later, so you can do things
like
%assign i i+1
to increment the numeric value of a macro.
%assign
is useful for controlling the termination of %rep preprocessor loops: see section
4.4 for an example of this. Another use for %assign is given in section 7.4 and section 8.1.
The expression passed to %assign is a critical expression (see section 3.7), and must also
evaluate to a pure number (rather than a relocatable reference such as a code or data
address, or anything involving a register).
4.2 Multi-Line Macros: %macro
Multi-line macros are much more like the type of macro seen in MASM and TASM: a
multi-line macro definition in NASM looks something like this.
%macro prologue 1
push ebp
mov ebp,esp
sub esp,%1
%endmacro
This defines a C-like function prologue as a macro: so you would invoke the macro with a
call such as
myfunc:
prologue 12
which would expand to the three lines of code
myfunc:
push ebp
mov ebp,esp
sub esp,12
The number 1 after the macro name in the %macro line defines the number of parameters
the macro prologue expects to receive. The use of %1 inside the macro definition refers to
the first parameter to the macro call. With a macro taking more than one parameter,
subsequent parameters would be referred to as %2, %3 and so on.
Multi-line macros, like single-line macros, are case-sensitive, unless you define them using
the alternative directive %imacro.
If you need to pass a comma as part of a parameter to a multi-line macro, you can do that
by enclosing the entire parameter in braces. So you could code things like
%macro silly 2
%2:
db %1
%endmacro
silly 'a', letter_a
silly 'ab', string_ab
silly {13,10}, crlf
; letter_a: db 'a'
; string_ab: db 'ab'
; crlf:
db 13,10
4.2.1 Overloading Multi-Line Macros
22
As with single-line macros, multi-line macros can be overloaded by defining the same
macro name several times with different numbers of parameters. This time, no exception is
made for macros with no parameters at all. So you could define
%macro prologue 0
push ebp
mov ebp,esp
%endmacro
to define an alternative form of the function prologue which allocates no local stack space.
Sometimes, however, you might want to `overload' a machine instruction; for example, you
might want to define
%macro push 2
push %1
push %2
%endmacro
so that you could code
push ebx
push eax,ecx
; this line is not a macro call
; but this one is
Ordinarily, NASM will give a warning for the first of the above two lines, since push is
now defined to be a macro, and is being invoked with a number of parameters for which no
definition has been given. The correct code will still be generated, but the assembler will
give a warning. This warning can be disabled by the use of the -w-macro-params
command-line option (see section 2.1.12).
4.2.2 Macro-Local Labels
NASM allows you to define labels within a multi-line macro definition in such a way as to
make them local to the macro call: so calling the same macro multiple times will use a
different label each time. You do this by prefixing %% to the label name. So you can invent
an instruction which executes a RET if the Z flag is set by doing this:
%macro retz 0
jnz %%skip
ret
%%skip:
%endmacro
You can call this macro as many times as you want, and every time you call it NASM will
make up a different `real' name to substitute for the label %%skip. The names NASM
invents are of the form ..@2345.skip, where the number 2345 changes with every macro
call. The ..@ prefix prevents macro-local labels from interfering with the local label
mechanism, as described in section 3.8. You should avoid defining your own labels in this
form (the ..@ prefix, then a number, then another period) in case they interfere with macrolocal labels.
4.2.3 Greedy Macro Parameters
Occasionally it is useful to define a macro which lumps its entire command line into one
parameter definition, possibly after extracting one or two smaller parameters from the front.
23
An example might be a macro to write a text string to a file in MS-DOS, where you might
want to be able to write
writefile [filehandle],"hello, world",13,10
NASM allows you to define the last parameter of a macro to be greedy, meaning that if you
invoke the macro with more parameters than it expects, all the spare parameters get lumped
into the last defined one along with the separating commas. So if you code:
%macro writefile 2+
jmp %%endstr
%%str:
db %2
%%endstr: mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
then the example call to writefile above will work as expected: the text before the first
comma, [filehandle], is used as the first macro parameter and expanded when %1 is
referred to, and all the subsequent text is lumped into %2 and placed after the db.
The greedy nature of the macro is indicated to NASM by the use of the + sign after the
parameter count on the %macro line.
If you define a greedy macro, you are effectively telling NASM how it should expand the
macro given any number of parameters from the actual number specified up to infinity; in
this case, for example, NASM now knows what to do when it sees a call to writefile with
2, 3, 4 or more parameters. NASM will take this into account when overloading macros,
and will not allow you to define another form of writefile taking 4 parameters (for
example).
Of course, the above macro could have been implemented as a non-greedy macro, in which
case the call to it would have had to look like
writefile [filehandle], {"hello, world",13,10}
NASM provides both mechanisms for putting commas in macro parameters, and you
choose which one you prefer for each macro definition.
See section 5.2.1 for a better way to write the above macro.
4.2.4 Default Macro Parameters
NASM also allows you to define a multi-line macro with a range of allowable parameter
counts. If you do this, you can specify defaults for omitted parameters. So, for example:
%macro die 0-1 "Painful program death has occurred."
writefile 2,%1
mov ax,0x4c01
int 0x21
%endmacro
This macro (which makes use of the writefile macro defined in section 4.2.3) can be
called with an explicit error message, which it will display on the error output stream before
24
exiting, or it can be called with no parameters, in which case it will use the default error
message supplied in the macro definition.
In general, you supply a minimum and maximum number of parameters for a macro of this
type; the minimum number of parameters are then required in the macro call, and then you
provide defaults for the optional ones. So if a macro definition began with the line
%macro foobar 1-3 eax,[ebx+2]
then it could be called with between one and three parameters, and %1 would always be
taken from the macro call. %2, if not specified by the macro call, would default to eax, and
%3 if not specified would default to [ebx+2].
You may omit parameter defaults from the macro definition, in which case the parameter
default is taken to be blank. This can be useful for macros which can take a variable
number of parameters, since the %0 token (see section 4.2.5) allows you to determine how
many parameters were really passed to the macro call.
This defaulting mechanism can be combined with the greedy-parameter mechanism; so the
die macro above could be made more powerful, and more useful, by changing the first line
of the definition to
%macro die 0-1+ "Painful program death has occurred.",13,10
The maximum parameter count can be infinite, denoted by *. In this case, of course, it is
impossible to provide a full set of default parameters. Examples of this usage are shown in
section 4.2.6.
4.2.5 %0: Macro Parameter Counter
For a macro which can take a variable number of parameters, the parameter reference %0
will return a numeric constant giving the number of parameters passed to the macro. This
can be used as an argument to %rep (see section 4.4) in order to iterate through all the
parameters of a macro. Examples are given in section 4.2.6.
4.2.6 %rotate: Rotating Macro Parameters
Unix shell programmers will be familiar with the shift shell command, which allows the
arguments passed to a shell script (referenced as $1, $2 and so on) to be moved left by one
place, so that the argument previously referenced as $2 becomes available as $1, and the
argument previously referenced as $1 is no longer available at all.
NASM provides a similar mechanism, in the form of %rotate. As its name suggests, it
differs from the Unix shift in that no parameters are lost: parameters rotated off the left
end of the argument list reappear on the right, and vice versa.
%rotate
is invoked with a single numeric argument (which may be an expression). The
macro parameters are rotated to the left by that many places. If the argument to %rotate is
negative, the macro parameters are rotated to the right.
So a pair of macros to save and restore a set of registers might work as follows:
%macro multipush 1-*
25
%rep %0
push %1
%rotate 1
%endrep
%endmacro
This macro invokes the PUSH instruction on each of its arguments in turn, from left to right.
It begins by pushing its first argument, %1, then invokes %rotate to move all the arguments
one place to the left, so that the original second argument is now available as %1. Repeating
this procedure as many times as there were arguments (achieved by supplying %0 as the
argument to %rep) causes each argument in turn to be pushed.
Note also the use of * as the maximum parameter count, indicating that there is no upper
limit on the number of parameters you may supply to the multipush macro.
It would be convenient, when using this macro, to have a POP equivalent, which didn't
require the arguments to be given in reverse order. Ideally, you would write the multipush
macro call, then cut-and-paste the line to where the pop needed to be done, and change the
name of the called macro to multipop, and the macro would take care of popping the
registers in the opposite order from the one in which they were pushed.
This can be done by the following definition:
%macro multipop 1-*
%rep %0
%rotate -1
pop %1
%endrep
%endmacro
This macro begins by rotating its arguments one place to the right, so that the original last
argument appears as %1. This is then popped, and the arguments are rotated right again, so
the second-to-last argument becomes %1. Thus the arguments are iterated through in reverse
order.
4.2.7 Concatenating Macro Parameters
NASM can concatenate macro parameters on to other text surrounding them. This allows
you to declare a family of symbols, for example, in a macro definition. If, for example, you
wanted to generate a table of key codes along with offsets into the table, you could code
something like
%macro keytab_entry 2
keypos%1 equ $-keytab
db %2
%endmacro
keytab:
keytab_entry F1,128+1
keytab_entry F2,128+2
keytab_entry Return,13
which would expand to
keytab:
keyposF1 equ $-keytab
26
db
keyposF2 equ
db
keyposReturn
db
128+1
$-keytab
128+2
equ $-keytab
13
You can just as easily concatenate text on to the other end of a macro parameter, by writing
%1foo.
If you need to append a digit to a macro parameter, for example defining labels foo1 and
foo2 when passed the parameter foo, you can't code %11 because that would be taken as
the eleventh macro parameter. Instead, you must code %{1}1, which will separate the first 1
(giving the number of the macro parameter) from the second (literal text to be concatenated
to the parameter).
This concatenation can also be applied to other preprocessor in-line objects, such as macrolocal labels (section 4.2.2) and context-local labels (section 4.6.2). In all cases, ambiguities
in syntax can be resolved by enclosing everything after the % sign and before the literal text
in braces: so %{%foo}bar concatenates the text bar to the end of the real name of the
macro-local label %%foo. (This is unnecessary, since the form NASM uses for the real
names of macro-local labels means that the two usages %{%foo}bar and %%foobar would
both expand to the same thing anyway; nevertheless, the capability is there.)
4.2.8 Condition Codes as Macro Parameters
NASM can give special treatment to a macro parameter which contains a condition code.
For a start, you can refer to the macro parameter %1 by means of the alternative syntax %+1,
which informs NASM that this macro parameter is supposed to contain a condition code,
and will cause the preprocessor to report an error message if the macro is called with a
parameter which is not a valid condition code.
Far more usefully, though, you can refer to the macro parameter by means of %-1, which
NASM will expand as the inverse condition code. So the retz macro defined in section
4.2.2 can be replaced by a general conditional-return macro like this:
%macro retc 1
j%-1 %%skip
ret
%%skip:
%endmacro
This macro can now be invoked using calls like retc ne, which will cause the conditionaljump instruction in the macro expansion to come out as JE, or retc po which will make
the jump a JPE.
The %+1 macro-parameter reference is quite happy to interpret the arguments CXZ and ECXZ
as valid condition codes; however, %-1 will report an error if passed either of these, because
no inverse condition code exists.
4.2.9 Disabling Listing Expansion
27
When NASM is generating a listing file from your program, it will generally expand multiline macros by means of writing the macro call and then listing each line of the expansion.
This allows you to see which instructions in the macro expansion are generating what code;
however, for some macros this clutters the listing up unnecessarily.
NASM therefore provides the .nolist qualifier, which you can include in a macro
definition to inhibit the expansion of the macro in the listing file. The .nolist qualifier
comes directly after the number of parameters, like this:
%macro foo 1.nolist
Or like this:
%macro bar 1-5+.nolist a,b,c,d,e,f,g,h
4.3 Conditional Assembly
Similarly to the C preprocessor, NASM allows sections of a source file to be assembled
only if certain conditions are met. The general syntax of this feature looks like this:
%if<condition>
; some code which only appears if <condition> is met
%elif<condition2>
; only appears if <condition> is not met but <condition2> is
%else
; this appears if neither <condition> nor <condition2> was met
%endif
The %else clause is optional, as is the %elif clause. You can have more than one %elif
clause as well.
4.3.1 %ifdef: Testing Single-Line Macro Existence
Beginning a conditional-assembly block with the line %ifdef MACRO will assemble the
subsequent code if, and only if, a single-line macro called MACRO is defined. If not, then the
%elif and %else blocks (if any) will be processed instead.
For example, when debugging a program, you might want to write code such as
; perform some function
%ifdef DEBUG
writefile 2,"Function performed successfully",13,10
%endif
; go and do something else
Then you could use the command-line option -dDEBUG to create a version of the program
which produced debugging messages, and remove the option to generate the final release
version of the program.
You can test for a macro not being defined by using %ifndef instead of %ifdef. You can
also test for macro definitions in %elif blocks by using %elifdef and %elifndef.
4.3.2 %ifctx: Testing the Context Stack
The conditional-assembly construct %ifctx ctxname will cause the subsequent code to be
assembled if and only if the top context on the preprocessor's context stack has the name
28
ctxname. As with %ifdef, the inverse and %elif
%elifnctx are also supported.
forms %ifnctx, %elifctx and
For more details of the context stack, see section 4.6. For a sample use of %ifctx, see
section 4.6.5.
4.3.3 %if: Testing Arbitrary Numeric Expressions
The conditional-assembly construct %if expr will cause the subsequent code to be
assembled if and only if the value of the numeric expression expr is non-zero. An example
of the use of this feature is in deciding when to break out of a %rep preprocessor loop: see
section 4.4 for a detailed example.
The expression given to %if, and its counterpart %elif, is a critical expression (see section
3.7).
%if
extends the normal NASM expression syntax, by providing a set of relational operators
which are not normally available in expressions. The operators =, <, >, <=, >= and <> test
equality, less-than, greater-than, less-or-equal, greater-or-equal and not-equal respectively.
The C-like forms == and != are supported as alternative forms of = and <>. In addition,
low-priority logical operators &&, ^^ and || are provided, supplying logical AND, logical
XOR and logical OR. These work like the C logical operators (although C has no logical
XOR), in that they always return either 0 or 1, and treat any non-zero input as 1 (so that ^^,
for example, returns 1 if exactly one of its inputs is zero, and 0 otherwise). The relational
operators also return 1 for true and 0 for false.
4.3.4 %ifidn and %ifidni: Testing Exact Text Identity
The construct %ifidn text1,text2 will cause the subsequent code to be assembled if and
only if text1 and text2, after expanding single-line macros, are identical pieces of text.
Differences in white space are not counted.
%ifidni
is similar to %ifidn, but is case-insensitive.
For example, the following macro pushes a register or number on the stack, and allows you
to treat IP as a real register:
%macro pushparam 1
%ifidni %1,ip
call %%label
%%label:
%else
push %1
%endif
%endmacro
Like most other %if constructs, %ifidn has a counterpart %elifidn, and negative forms
%ifnidn and %elifnidn. Similarly, %ifidni has counterparts %elifidni, %ifnidni and
%elifnidni.
4.3.5 %ifid, %ifnum, %ifstr: Testing Token Types
29
Some macros will want to perform different tasks depending on whether they are passed a
number, a string, or an identifier. For example, a string output macro might want to be able
to cope with being passed either a string constant or a pointer to an existing string.
The conditional assembly construct %ifid, taking one parameter (which may be blank),
assembles the subsequent code if and only if the first token in the parameter exists and is an
identifier. %ifnum works similarly, but tests for the token being a numeric constant; %ifstr
tests for it being a string.
For example, the writefile macro defined in section 4.2.3 can be extended to take
advantage of %ifstr in the following fashion:
%macro writefile 2-3+
%ifstr %2
jmp %%endstr
%if %0 = 3
%%str:
db %2,%3
%else
%%str:
db %2
%endif
%%endstr: mov dx,%%str
mov cx,%%endstr-%%str
%else
mov dx,%2
mov cx,%3
%endif
mov bx,%1
mov ah,0x40
int 0x21
%endmacro
Then the writefile macro can cope with being called in either of the following two ways:
writefile [file], strpointer, length
writefile [file], "hello", 13, 10
In the first, strpointer is used as the address of an already-declared string, and length is
used as its length; in the second, a string is given to the macro, which therefore declares it
itself and works out the address and length for itself.
Note the use of %if inside the %ifstr: this is to detect whether the macro was passed two
arguments (so the string would be a single string constant, and db %2 would be adequate)
or more (in which case, all but the first two would be lumped together into %3, and db
%2,%3 would be required).
The usual %elifXXX, %ifnXXX and %elifnXXX versions exist for each of %ifid, %ifnum
and %ifstr.
4.3.6 %error: Reporting User-Defined Errors
The preprocessor directive %error will cause NASM to report an error if it occurs in
assembled code. So if other users are going to try to assemble your source files, you can
ensure that they define the right macros by means of code like this:
%ifdef SOME_MACRO
30
; do some setup
%elifdef SOME_OTHER_MACRO
; do some different setup
%else
%error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined.
%endif
Then any user who fails to understand the way your code is supposed to be assembled will
be quickly warned of their mistake, rather than having to wait until the program crashes on
being run and then not knowing what went wrong.
4.4 Preprocessor Loops: %rep
NASM's TIMES prefix, though useful, cannot be used to invoke a multi-line macro multiple
times, because it is processed by NASM after macros have already been expanded.
Therefore NASM provides another form of loop, this time at the preprocessor level: %rep.
The directives %rep and %endrep (%rep takes a numeric argument, which can be an
expression; %endrep takes no arguments) can be used to enclose a chunk of code, which is
then replicated as many times as specified by the preprocessor:
%assign i 0
%rep 64
inc word [table+2*i]
%assign i i+1
%endrep
This will generate a sequence of 64 INC instructions, incrementing every word of memory
from [table] to [table+126].
For more complex termination conditions, or to break out of a repeat loop part way along,
you can use the %exitrep directive to terminate the loop, like this:
fibonacci:
%assign i 0
%assign j 1
%rep 100
%if j > 65535
%exitrep
%endif
dw j
%assign k j+i
%assign i j
%assign j k
%endrep
fib_number equ ($-fibonacci)/2
This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a
maximum repeat count must still be given to %rep. This is to prevent the possibility of
NASM getting into an infinite loop in the preprocessor, which (on multitasking or multiuser systems) would typically cause all the system memory to be gradually used up and
other applications to start crashing.
4.5 Including Other Files
31
Using, once again, a very similar syntax to the C preprocessor, NASM's preprocessor lets
you include other source files into your code. This is done by the use of the %include
directive:
%include "macros.mac"
will include the contents of the file macros.mac into the source file containing the
%include directive.
Include files are searched for in the current directory (the directory you're in when you run
NASM, as opposed to the location of the NASM executable or the location of the source
file), plus any directories specified on the NASM command line using the -i option.
The standard C idiom for preventing a file being included more than once is just as
applicable in NASM: if the file macros.mac has the form
%ifndef MACROS_MAC
%define MACROS_MAC
; now define some macros
%endif
then including the file more than once will not cause errors, because the second time the
file is included nothing will happen because the macro MACROS_MAC will already be defined.
You can force a file to be included even if there is no %include directive that explicitly
includes it, by using the -p option on the NASM command line (see section 2.1.7).
4.6 The Context Stack
Having labels that are local to a macro definition is sometimes not quite powerful enough:
sometimes you want to be able to share labels between several macro calls. An example
might be a REPEAT ... UNTIL loop, in which the expansion of the REPEAT macro would need
to be able to refer to a label which the UNTIL macro had defined. However, for such a
macro you would also want to be able to nest these loops.
NASM provides this level of power by means of a context stack. The preprocessor
maintains a stack of contexts, each of which is characterised by a name. You add a new
context to the stack using the %push directive, and remove one using %pop. You can define
labels that are local to a particular context on the stack.
4.6.1 %push and %pop: Creating and Removing Contexts
The %push directive is used to create a new context and place it on the top of the context
stack. %push requires one argument, which is the name of the context. For example:
%push foobar
This pushes a new context called foobar on the stack. You can have several contexts on
the stack with the same name: they can still be distinguished.
The directive %pop, requiring no arguments, removes the top context from the context stack
and destroys it, along with any labels associated with it.
4.6.2 Context-Local Labels
32
Just as the usage %%foo defines a label which is local to the particular macro call in which it
is used, the usage %$foo is used to define a label which is local to the context on the top of
the context stack. So the REPEAT and UNTIL example given above could be implemented by
means of:
%macro repeat 0
%push repeat
%$begin:
%endmacro
%macro until 1
j%-1 %$begin
%pop
%endmacro
and invoked by means of, for example,
mov cx,string
repeat
add cx,3
scasb
until e
which would scan every fourth byte of a string in search of the byte in AL.
If you need to define, or access, labels local to the context below the top one on the stack,
you can use %$$foo, or %$$$foo for the context below that, and so on.
4.6.3 Context-Local Single-Line Macros
NASM also allows you to define single-line macros which are local to a particular context,
in just the same way:
%define %$localmac 3
will define the single-line macro %$localmac to be local to the top context on the stack. Of
course, after a subsequent %push, it can then still be accessed by the name %$$localmac.
4.6.4 %repl: Renaming a Context
If you need to change the name of the top context on the stack (in order, for example, to
have it respond differently to %ifctx), you can execute a %pop followed by a %push; but
this will have the side effect of destroying all context-local labels and macros associated
with the context that was just popped.
NASM provides the directive %repl, which replaces a context with a different name,
without touching the associated macros and labels. So you could replace the destructive
code
%pop
%push newname
with the non-destructive version %repl newname.
4.6.5 Example Use of the Context Stack: Block IFs
33
This example makes use of almost all the context-stack features, including the conditionalassembly construct %ifctx, to implement a block IF statement as a set of macros.
%macro if 1
%push if
j%-1 %$ifnot
%endmacro
%macro else 0
%ifctx if
%repl else
jmp %$ifend
%$ifnot:
%else
%error "expected `if' before `else'"
%endif
%endmacro
%macro endif 0
%ifctx if
%$ifnot:
%pop
%elifctx else
%$ifend:
%pop
%else
%error "expected `if' or `else' before `endif'"
%endif
%endmacro
This code is more robust than the REPEAT and UNTIL macros given in section 4.6.2, because
it uses conditional assembly to check that the macros are issued in the right order (for
example, not calling endif before if) and issues a %error if they're not.
In addition, the endif macro has to be able to cope with the two distinct cases of either
directly following an if, or following an else. It achieves this, again, by using conditional
assembly to do different things depending on whether the context on top of the stack is if
or else.
The else macro has to preserve the context on the stack, in order to have the %$ifnot
referred to by the if macro be the same as the one defined by the endif macro, but has to
change the context's name so that endif will know there was an intervening else. It does
this by the use of %repl.
A sample usage of these macros might look like:
cmp ax,bx
if ae
cmp bx,cx
if ae
mov ax,cx
else
mov ax,bx
endif
else
cmp ax,cx
if ae
34
mov ax,cx
endif
endif
The block-IF macros handle nesting quite happily, by means of pushing another context,
describing the inner if, on top of the one describing the outer if; thus else and endif
always refer to the last unmatched if or else.
4.7 Standard Macros
NASM defines a set of standard macros, which are already defined when it starts to process
any source file. If you really need a program to be assembled with no pre-defined macros,
you can use the %clear directive to empty the preprocessor of everything.
Most user-level assembler directives (see chapter 5) are implemented as macros which
invoke primitive directives; these are described in chapter 5. The rest of the standard macro
set is described here.
4.7.1 __NASM_MAJOR__ and __NASM_MINOR__: NASM Version
The single-line macros __NASM_MAJOR__ and __NASM_MINOR__ expand to the major and
minor parts of the version number of NASM being used. So, under NASM 0.96 for
example, __NASM_MAJOR__ would be defined to be 0 and __NASM_MINOR__ would be
defined as 96.
4.7.2 __FILE__ and __LINE__: File Name and Line Number
Like the C preprocessor, NASM allows the user to find out the file name and line number
containing the current instruction. The macro __FILE__ expands to a string constant giving
the name of the current input file (which may change through the course of assembly if
%include directives are used), and __LINE__ expands to a numeric constant giving the
current line number in the input file.
These macros could be used, for example, to communicate debugging information to a
macro, since invoking __LINE__ inside a macro definition (either single-line or multi-line)
will return the line number of the macro call, rather than definition. So to determine where
in a piece of code a crash is occurring, for example, one could write a routine stillhere,
which is passed a line number in EAX and outputs something like `line 155: still here'. You
could then write a macro
%macro notdeadyet 0
push eax
mov eax,__LINE__
call stillhere
pop eax
%endmacro
and then pepper your code with calls to notdeadyet until you find the crash point.
4.7.3 STRUC and ENDSTRUC: Declaring Structure Data Types
35
The core of NASM contains no intrinsic means of defining data structures; instead, the
preprocessor is sufficiently powerful that data structures can be implemented as a set of
macros. The macros STRUC and ENDSTRUC are used to define a structure data type.
STRUC
takes one parameter, which is the name of the data type. This name is defined as a
symbol with the value zero, and also has the suffix _size appended to it and is then defined
as an EQU giving the size of the structure. Once STRUC has been issued, you are defining the
structure, and should define fields using the RESB family of pseudo-instructions, and then
invoke ENDSTRUC to finish the definition.
For example, to define a structure called mytype containing a longword, a word, a byte and
a string of bytes, you might code
mt_long:
mt_word:
mt_byte:
mt_str:
struc mytype
resd 1
resw 1
resb 1
resb 32
endstruc
The above code defines six symbols: mt_long as 0 (the offset from the beginning of a
mytype structure to the longword field), mt_word as 4, mt_byte as 6, mt_str as 7,
mytype_size as 39, and mytype itself as zero.
The reason why the structure type name is defined at zero is a side effect of allowing
structures to work with the local label mechanism: if your structure members tend to have
the same names in more than one structure, you can define the above structure like this:
.long:
.word:
.byte:
.str:
struc mytype
resd 1
resw 1
resb 1
resb 32
endstruc
This defines the offsets to the structure fields as mytype.long, mytype.word,
mytype.byte and mytype.str.
NASM, since it has no intrinsic structure support, does not support any form of period
notation to refer to the elements of a structure once you have one (except the above locallabel notation), so code such as mov ax,[mystruc.mt_word] is not valid. mt_word is a
constant just like any other constant, so the correct syntax is mov ax,[mystruc+mt_word]
or mov ax,[mystruc+mytype.word].
4.7.4 ISTRUC, AT and IEND: Declaring Instances of Structures
Having defined a structure type, the next thing you typically want to do is to declare
instances of that structure in your data segment. NASM provides an easy way to do this in
the ISTRUC mechanism. To declare a structure of type mytype in a program, you code
something like this:
mystruc:
istruc mytype
at mt_long, dd 123456
at mt_word, dw 1024
36
at mt_byte, db 'x'
at mt_str, db 'hello, world', 13, 10, 0
iend
The function of the AT macro is to make use of the TIMES prefix to advance the assembly
position to the correct point for the specified structure field, and then to declare the
specified data. Therefore the structure fields must be declared in the same order as they
were specified in the structure definition.
If the data to go in a structure field requires more than one source line to specify, the
remaining source lines can easily come after the AT line. For example:
at mt_str, db 123,134,145,156,167,178,189
db 190,100,0
Depending on personal taste, you can also omit the code part of the AT line completely, and
start the structure field on the next line:
at mt_str
db 'hello, world'
db 13,10,0
4.7.5 ALIGN and ALIGNB: Data Alignment
The ALIGN and ALIGNB macros provides a convenient way to align code or data on a word,
longword, paragraph or other boundary. (Some assemblers call this directive EVEN.) The
syntax of the ALIGN and ALIGNB macros is
align 4
align 16
align 8,db 0
align 4,resb 1
alignb 4
;
;
;
;
;
align on 4-byte boundary
align on 16-byte boundary
pad with 0s rather than NOPs
align to 4 in the BSS
equivalent to previous line
Both macros require their first argument to be a power of two; they both compute the
number of additional bytes required to bring the length of the current section up to a
multiple of that power of two, and then apply the TIMES prefix to their second argument to
perform the alignment.
If the second argument is not specified, the default for ALIGN is NOP, and the default for
ALIGNB is RESB 1. So if the second argument is specified, the two macros are equivalent.
Normally, you can just use ALIGN in code and data sections and ALIGNB in BSS sections,
and never need the second argument except for special purposes.
ALIGN
and ALIGNB, being simple macros, perform no error checking: they cannot warn you
if their first argument fails to be a power of two, or if their second argument generates more
than one byte of code. In each of these cases they will silently do the wrong thing.
ALIGNB
(or ALIGN with a second argument of RESB 1) can be used within structure
definitions:
mt_byte:
mt_word:
struc mytype2
resb 1
alignb 2
resw 1
alignb 4
37
mt_long:
mt_str:
resd 1
resb 32
endstruc
This will ensure that the structure members are sensibly aligned relative to the base of the
structure.
A final caveat: ALIGN and ALIGNB work relative to the beginning of the section, not the
beginning of the address space in the final executable. Aligning to a 16-byte boundary
when the section you're in is only guaranteed to be aligned to a 4-byte boundary, for
example, is a waste of effort. Again, NASM does not check that the section's alignment
characteristics are sensible for the use of ALIGN or ALIGNB.
Chapter 5: Assembler Directives
NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM,
is nevertheless forced to support a few directives. These are described in this chapter.
NASM's directives come in two types: user-level directivesuser-level directives and
primitive directivesprimitive directives. Typically, each directive has a user-level form and
a primitive form. In almost all cases, we recommend that users use the user-level forms of
the directives, which are implemented as macros which call the primitive forms.
Primitive directives are enclosed in square brackets; user-level directives are not.
In addition to the universal directives described in this chapter, each object file format can
optionally supply extra directives in order to control particular features of that file format.
These format-specific directivesformat-specific directives are documented along with the
formats that implement them, in chapter 6.
5.1 BITS: Specifying Target Processor Mode
The BITS directive specifies whether NASM should generate code designed to run on a
processor operating in 16-bit mode, or code designed to run on a processor operating in 32bit mode. The syntax is BITS 16 or BITS 32.
In most cases, you should not need to use BITS explicitly. The aout, coff, elf and win32
object formats, which are designed for use in 32-bit operating systems, all cause NASM to
select 32-bit mode by default. The obj object format allows you to specify each segment
you define as either USE16 or USE32, and NASM will set its operating mode accordingly, so
the use of the BITS directive is once again unnecessary.
The most likely reason for using the BITS directive is to write 32-bit code in a flat binary
file; this is because the bin output format defaults to 16-bit mode in anticipation of it being
used most frequently to write DOS .COM programs, DOS .SYS device drivers and boot
loader software.
38
You do not need to specify BITS 32 merely in order to use 32-bit instructions in a 16-bit
DOS program; if you do, the assembler will generate incorrect code because it will be
writing code targeted at a 32-bit platform, to be run on a 16-bit one.
When NASM is in BITS 16 state, instructions which use 32-bit data are prefixed with an
0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In BITS 32 state,
the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit
data need an 0x66 and those working in 16-bit addresses need an 0x67.
The BITS directive has an exactly equivalent primitive form, [BITS 16] and [BITS 32].
The user-level form is a macro which has no function other than to call the primitive form.
5.2 SECTION or SEGMENT: Changing and Defining Sections
The SECTION directive (SEGMENT is an exactly equivalent synonym) changes which section
of the output file the code you write will be assembled into. In some object file formats, the
number and names of sections are fixed; in others, the user may make up as many as they
wish. Hence SECTION may sometimes give an error message, or may define a new section,
if you try to switch to a section that does not (yet) exist.
The Unix object formats, and the bin object format, all support the standardised section
names .text, .data and .bss for the code, data and uninitialised-data sections. The obj
format, by contrast, does not recognise these section names as being special, and indeed
will strip off the leading period of any section name that has one.
5.2.1 The __SECT__ Macro
The SECTION directive is unusual in that its user-level form functions differently from its
primitive form. The primitive form, [SECTION xyz], simply switches the current target
section to the one given. The user-level form, SECTION xyz, however, first defines the
single-line macro __SECT__ to be the primitive [SECTION] directive which it is about to
issue, and then issues it. So the user-level directive
SECTION .text
expands to the two lines
%define __SECT__ [SECTION .text]
[SECTION .text]
Users may find it useful to make use of this in their own macros. For example, the
writefile macro defined in section 4.2.3 can be usefully rewritten in the following more
sophisticated form:
%macro writefile 2+
[section .data]
%%str:
db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
39
%endmacro
This form of the macro, once passed a string to output, first switches temporarily to the data
section of the file, using the primitive form of the SECTION directive so as not to modify
__SECT__. It then declares its string in the data section, and then invokes __SECT__ to
switch back to whichever section the user was previously working in. It thus avoids the
need, in the previous version of the macro, to include a JMP instruction to jump over the
data, and also does not fail if, in a complicated OBJ format module, the user could
potentially be assembling the code in any of several separate code sections.
5.3 ABSOLUTE: Defining Absolute Labels
The ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes the
subsequent code to be directed at no physical section, but at the hypothetical section
starting at the given absolute address. The only instructions you can use in this mode are
the RESB family.
ABSOLUTE
is used as follows:
absolute 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf
resw 16
This example describes a section of the PC BIOS data area, at segment address 0x40: the
above code defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.
The user-level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro
when it is invoked.
STRUC
and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).
ABSOLUTE
doesn't have to take an absolute constant as an argument: it can take an
expression (actually, a critical expression: see section 3.7) and it can be a value in a
segment. For example, a TSR can re-use its setup code as run-time BSS like this:
org 100h
; it's a .COM program
jmp setup
; setup code comes last
; the resident part of the TSR goes here
setup:
; now write the code that installs the TSR here
absolute setup
runtimevar1 resw 1
runtimevar2 resd 20
tsr_end:
This defines some variables `on top of' the setup code, so that after the setup has finished
running, the space it took up can be re-used as data storage for the running TSR. The
symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to
be made resident.
5.4 EXTERN: Importing Symbols from Other Modules
EXTERN
is similar to the MASM directive EXTRN and the C keyword extern: it is used to
declare a symbol which is not defined anywhere in the module being assembled, but is
40
assumed to be defined in some other module and needs to be referred to by this one. Not
every object-file format can support external variables: the bin format cannot.
The EXTERN directive takes as many arguments as you like. Each argument is the name of a
symbol:
extern _printf
extern _sscanf,_fscanf
Some object-file formats provide extra features to the EXTERN directive. In all cases, the
extra features are used by suffixing a colon to the symbol name followed by object-format
specific text. For example, the obj format allows you to declare that the default segment
base of an external should be the group dgroup by means of the directive
extern _variable:wrt dgroup
The primitive form of EXTERN differs from the user-level form only in that it can take only
one argument at a time: the support for multiple arguments is implemented at the
preprocessor level.
You can declare the same variable as EXTERN more than once: NASM will quietly ignore
the second and later redeclarations. You can't declare a variable as EXTERN as well as
something else, though.
5.5 GLOBAL: Exporting Symbols to Other Modules
GLOBAL
is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to
it, then in order to prevent linker errors, some other module must actually define the symbol
and declare it as GLOBAL. Some assemblers use the name PUBLIC for this purpose.
The GLOBAL directive applying to a symbol must appear before the definition of the symbol.
GLOBAL
uses the same syntax as EXTERN, except that it must refer to symbols which are
defined in the same module as the GLOBAL directive. For example:
_main:
global _main
; some code
GLOBAL,
like EXTERN, allows object formats to define private extensions by means of a
colon. The elf object format, for example, lets you specify whether global data items are
functions or data:
global hashlookup:function, hashtable:data
Like EXTERN, the primitive form of GLOBAL differs from the user-level form only in that it
can take only one argument at a time.
5.6 COMMON: Defining Common Data Areas
The COMMON directive is used to declare common variables. A common variable is much
like a global variable declared in the uninitialised data section, so that
common intvar 4
is similar in function to
global intvar
41
intvar
section .bss
resd 1
The difference is that if more than one module defines the same common variable, then at
link time those variables will be merged, and references to intvar in all modules will point
at the same piece of memory.
Like GLOBAL and EXTERN, COMMON supports object-format specific extensions. For example,
the obj format allows common variables to be NEAR or FAR, and the elf format allows
you to specify the alignment requirements of a common variable:
common commvar 4:near
common intarray 100:4
; works in OBJ
; works in ELF: 4 byte aligned
Once again, like EXTERN and GLOBAL, the primitive form of COMMON differs from the userlevel form only in that it can take only one argument at a time.
Chapter 5: Assembler Directives
NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM,
is nevertheless forced to support a few directives. These are described in this chapter.
NASM's directives come in two types: user-level directivesuser-level directives and
primitive directivesprimitive directives. Typically, each directive has a user-level form and
a primitive form. In almost all cases, we recommend that users use the user-level forms of
the directives, which are implemented as macros which call the primitive forms.
Primitive directives are enclosed in square brackets; user-level directives are not.
In addition to the universal directives described in this chapter, each object file format can
optionally supply extra directives in order to control particular features of that file format.
These format-specific directivesformat-specific directives are documented along with the
formats that implement them, in chapter 6.
5.1 BITS: Specifying Target Processor Mode
The BITS directive specifies whether NASM should generate code designed to run on a
processor operating in 16-bit mode, or code designed to run on a processor operating in 32bit mode. The syntax is BITS 16 or BITS 32.
In most cases, you should not need to use BITS explicitly. The aout, coff, elf and win32
object formats, which are designed for use in 32-bit operating systems, all cause NASM to
select 32-bit mode by default. The obj object format allows you to specify each segment
you define as either USE16 or USE32, and NASM will set its operating mode accordingly, so
the use of the BITS directive is once again unnecessary.
The most likely reason for using the BITS directive is to write 32-bit code in a flat binary
file; this is because the bin output format defaults to 16-bit mode in anticipation of it being
used most frequently to write DOS .COM programs, DOS .SYS device drivers and boot
loader software.
42
You do not need to specify BITS 32 merely in order to use 32-bit instructions in a 16-bit
DOS program; if you do, the assembler will generate incorrect code because it will be
writing code targeted at a 32-bit platform, to be run on a 16-bit one.
When NASM is in BITS 16 state, instructions which use 32-bit data are prefixed with an
0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In BITS 32 state,
the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit
data need an 0x66 and those working in 16-bit addresses need an 0x67.
The BITS directive has an exactly equivalent primitive form, [BITS 16] and [BITS 32].
The user-level form is a macro which has no function other than to call the primitive form.
5.2 SECTION or SEGMENT: Changing and Defining Sections
The SECTION directive (SEGMENT is an exactly equivalent synonym) changes which section
of the output file the code you write will be assembled into. In some object file formats, the
number and names of sections are fixed; in others, the user may make up as many as they
wish. Hence SECTION may sometimes give an error message, or may define a new section,
if you try to switch to a section that does not (yet) exist.
The Unix object formats, and the bin object format, all support the standardised section
names .text, .data and .bss for the code, data and uninitialised-data sections. The obj
format, by contrast, does not recognise these section names as being special, and indeed
will strip off the leading period of any section name that has one.
5.2.1 The __SECT__ Macro
The SECTION directive is unusual in that its user-level form functions differently from its
primitive form. The primitive form, [SECTION xyz], simply switches the current target
section to the one given. The user-level form, SECTION xyz, however, first defines the
single-line macro __SECT__ to be the primitive [SECTION] directive which it is about to
issue, and then issues it. So the user-level directive
SECTION .text
expands to the two lines
%define __SECT__ [SECTION .text]
[SECTION .text]
Users may find it useful to make use of this in their own macros. For example, the
writefile macro defined in section 4.2.3 can be usefully rewritten in the following more
sophisticated form:
%macro writefile 2+
[section .data]
%%str:
db %2
%%endstr:
__SECT__
mov dx,%%str
mov cx,%%endstr-%%str
mov bx,%1
mov ah,0x40
int 0x21
43
%endmacro
This form of the macro, once passed a string to output, first switches temporarily to the data
section of the file, using the primitive form of the SECTION directive so as not to modify
__SECT__. It then declares its string in the data section, and then invokes __SECT__ to
switch back to whichever section the user was previously working in. It thus avoids the
need, in the previous version of the macro, to include a JMP instruction to jump over the
data, and also does not fail if, in a complicated OBJ format module, the user could
potentially be assembling the code in any of several separate code sections.
5.3 ABSOLUTE: Defining Absolute Labels
The ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes the
subsequent code to be directed at no physical section, but at the hypothetical section
starting at the given absolute address. The only instructions you can use in this mode are
the RESB family.
ABSOLUTE
is used as follows:
absolute 0x1A
kbuf_chr resw 1
kbuf_free resw 1
kbuf
resw 16
This example describes a section of the PC BIOS data area, at segment address 0x40: the
above code defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.
The user-level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro
when it is invoked.
STRUC
and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).
ABSOLUTE
doesn't have to take an absolute constant as an argument: it can take an
expression (actually, a critical expression: see section 3.7) and it can be a value in a
segment. For example, a TSR can re-use its setup code as run-time BSS like this:
org 100h
; it's a .COM program
jmp setup
; setup code comes last
; the resident part of the TSR goes here
setup:
; now write the code that installs the TSR here
absolute setup
runtimevar1 resw 1
runtimevar2 resd 20
tsr_end:
This defines some variables `on top of' the setup code, so that after the setup has finished
running, the space it took up can be re-used as data storage for the running TSR. The
symbol `tsr_end' can be used to calculate the total size of the part of the TSR that needs to
be made resident.
5.4 EXTERN: Importing Symbols from Other Modules
EXTERN
is similar to the MASM directive EXTRN and the C keyword extern: it is used to
declare a symbol which is not defined anywhere in the module being assembled, but is
44
assumed to be defined in some other module and needs to be referred to by this one. Not
every object-file format can support external variables: the bin format cannot.
The EXTERN directive takes as many arguments as you like. Each argument is the name of a
symbol:
extern _printf
extern _sscanf,_fscanf
Some object-file formats provide extra features to the EXTERN directive. In all cases, the
extra features are used by suffixing a colon to the symbol name followed by object-format
specific text. For example, the obj format allows you to declare that the default segment
base of an external should be the group dgroup by means of the directive
extern _variable:wrt dgroup
The primitive form of EXTERN differs from the user-level form only in that it can take only
one argument at a time: the support for multiple arguments is implemented at the
preprocessor level.
You can declare the same variable as EXTERN more than once: NASM will quietly ignore
the second and later redeclarations. You can't declare a variable as EXTERN as well as
something else, though.
5.5 GLOBAL: Exporting Symbols to Other Modules
GLOBAL
is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to
it, then in order to prevent linker errors, some other module must actually define the symbol
and declare it as GLOBAL. Some assemblers use the name PUBLIC for this purpose.
The GLOBAL directive applying to a symbol must appear before the definition of the symbol.
GLOBAL
uses the same syntax as EXTERN, except that it must refer to symbols which are
defined in the same module as the GLOBAL directive. For example:
_main:
global _main
; some code
GLOBAL,
like EXTERN, allows object formats to define private extensions by means of a
colon. The elf object format, for example, lets you specify whether global data items are
functions or data:
global hashlookup:function, hashtable:data
Like EXTERN, the primitive form of GLOBAL differs from the user-level form only in that it
can take only one argument at a time.
5.6 COMMON: Defining Common Data Areas
The COMMON directive is used to declare common variables. A common variable is much
like a global variable declared in the uninitialised data section, so that
common intvar 4
is similar in function to
global intvar
45
intvar
section .bss
resd 1
The difference is that if more than one module defines the same common variable, then at
link time those variables will be merged, and references to intvar in all modules will point
at the same piece of memory.
Like GLOBAL and EXTERN, COMMON supports object-format specific extensions. For example,
the obj format allows common variables to be NEAR or FAR, and the elf format allows
you to specify the alignment requirements of a common variable:
common commvar 4:near
common intarray 100:4
; works in OBJ
; works in ELF: 4 byte aligned
Once again, like EXTERN and GLOBAL, the primitive form of COMMON differs from the userlevel form only in that it can take only one argument at a time.
Chapter 6: Output Formats
NASM is a portable assembler, designed to be able to compile on any ANSI C-supporting
platform and produce output to run on a variety of Intel x86 operating systems. For this
reason, it has a large number of available output formats, selected using the -f option on
the NASM command line. Each of these formats, along with its extensions to the base
NASM syntax, is detailed in this chapter.
As stated in section 2.1.1, NASM chooses a default name for your output file based on the
input file name and the chosen output format. This will be generated by removing the
extension (.asm, .s, or whatever you like to use) from the input file name, and substituting
an extension defined by the output format. The extensions are given with each format
below.
6.1 bin: Flat-Form Binary Output
The bin format does not produce object files: it generates nothing in the output file except
the code you wrote. Such `pure binary' files are used by MS-DOS: .COM executables and
.SYS device drivers are pure binary files. Pure binary output is also useful for operatingsystem and boot loader development.
bin
supports the three standardised section names .text, .data and .bss only. The file
NASM outputs will contain the contents of the .text section first, followed by the contents
of the .data section, aligned on a four-byte boundary. The .bss section is not stored in the
output file at all, but is assumed to appear directly after the end of the .data section, again
aligned on a four-byte boundary.
If you specify no explicit SECTION directive, the code you write will be directed by default
into the .text section.
46
Using the bin format puts NASM by default into 16-bit mode (see section 5.1). In order to
use bin to write 32-bit code such as an OS kernel, you need to explicitly issue the BITS 32
directive.
bin
has no default output file name extension: instead, it leaves your file name as it is once
the original extension has been removed. Thus, the default is for NASM to assemble
binprog.asm into a binary file called binprog.
6.1.1 ORG: Binary File Program Origin
The bin format provides an additional directive to the list given in chapter 5: ORG. The
function of the ORG directive is to specify the origin address which NASM will assume the
program begins at when it is loaded into memory.
For example, the following code will generate the longword 0x00000104:
org 0x100
dd label
label:
Unlike the ORG directive provided by MASM-compatible assemblers, which allows you to
jump around in the object file and overwrite code you have already generated, NASM's ORG
does exactly what the directive says: origin. Its sole function is to specify one offset which
is added to all internal address references within the file; it does not permit any of the
trickery that MASM's version does. See section 10.1.3 for further comments.
6.1.2 bin Extensions to the SECTION Directive
The bin output format extends the SECTION (or SEGMENT) directive to allow you to specify
the alignment requirements of segments. This is done by appending the ALIGN qualifier to
the end of the section-definition line. For example,
section .data align=16
switches to the section .data and also specifies that it must be aligned on a 16-byte
boundary.
The parameter to ALIGN specifies how many low bits of the section start address must be
forced to zero. The alignment value given may be any power of two.
6.2 obj: Microsoft OMF Object Files
The obj file format (NASM calls it obj rather than omf for historical reasons) is the one
produced by MASM and TASM, which is typically fed to 16-bit DOS linkers to produce
.EXE files. It is also the format used by OS/2.
obj
provides a default output file-name extension of .obj.
obj
is not exclusively a 16-bit format, though: NASM has full support for the 32-bit
extensions to the format. In particular, 32-bit obj format files are used by Borland's Win32
compilers, instead of using Microsoft's newer win32 object file format.
47
The obj format does not define any special segment names: you can call your segments
anything you like. Typical names for segments in obj format files are CODE, DATA and BSS.
If your source file contains code before specifying an explicit SEGMENT directive, then
NASM will invent its own segment called __NASMDEFSEG for you.
When you define a segment in an obj file, NASM defines the segment name as a symbol as
well, so that you can access the segment address of the segment. So, for example:
segment data
dw 1234
segment code
function: mov ax,data
mov ds,ax
inc word [dvar]
ret
dvar:
; get segment address of data
; and move it into DS
; now this reference will work
The obj format also enables the use of the SEG and WRT operators, so that you can write
code which does things like
extern foo
mov ax,seg foo
; get preferred segment of foo
mov ds,ax
mov ax,data
; a different segment
mov es,ax
mov ax,[ds:foo]
; this accesses `foo'
mov [es:foo wrt data],bx ; so does this
6.2.1 obj Extensions to the SEGMENT Directive
The obj output format extends the SEGMENT (or SECTION) directive to allow you to specify
various properties of the segment you are defining. This is done by appending extra
qualifiers to the end of the segment-definition line. For example,
segment code private align=16
defines the segment code, but also declares it to be a private segment, and requires that the
portion of it described in this code module must be aligned on a 16-byte boundary.
The available qualifiers are:

PRIVATE, PUBLIC, COMMON and STACK specify the combination characteristics of the segment.
PRIVATE segments do not get combined with any others by the linker; PUBLIC and STACK
segments get concatenated together at link time; and COMMON segments all get overlaid on top of
each other rather than stuck end-to-end.

ALIGN is used, as shown above, to specify how many low bits of the segment start address must be
forced to zero. The alignment value given may be any power of two from 1 to 4096; in reality, the
only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is specified it will be rounded up to 16,
and 32, 64 and 128 will all be rounded up to 256, and so on. Note that alignment to 4096-byte
boundaries is a PharLap extension to the format and may not be supported by all linkers.

CLASS can be used to specify the segment class; this feature indicates to the linker that segments of
the same class should be placed near each other in the output file. The class name can be any word,
e.g. CLASS=CODE.
48

OVERLAY, like CLASS, is specified with an arbitrary word as an argument, and provides overlay
information to an overlay-capable linker.

Segments can be declared as USE16 or USE32, which has the effect of recording the choice in the
object file and also ensuring that NASM's default assembly mode when assembling in that segment
is 16-bit or 32-bit respectively.

When writing OS/2 object files, you should declare 32-bit segments as FLAT, which causes the
default segment base for anything in the segment to be the special group FLAT, and also defines the
group if it is not already defined.

The obj file format also allows segments to be declared as having a pre-defined absolute segment
address, although no linkers are currently known to make sensible use of this feature; nevertheless,
NASM allows you to declare a segment such as SEGMENT SCREEN ABSOLUTE=0xB800 if you
need to. The ABSOLUTE and ALIGN keywords are mutually exclusive.
NASM's default segment attributes are PUBLIC, ALIGN=1, no class, no overlay, and USE16.
6.2.2 GROUP: Defining Groups of Segments
The obj format also allows segments to be grouped, so that a single segment register can be
used to refer to all the segments in a group. NASM therefore supplies the GROUP directive,
whereby you can code
segment data
; some data
segment bss
; some uninitialised data
group dgroup data bss
which will define a group called dgroup to contain the segments data and bss. Like
SEGMENT, GROUP causes the group name to be defined as a symbol, so that you can refer to a
variable var in the data segment as var wrt data or as var wrt dgroup, depending on
which segment value is currently in your segment register.
If you just refer to var, however, and var is declared in a segment which is part of a group,
then NASM will default to giving you the offset of var from the beginning of the group,
not the segment. Therefore SEG var, also, will return the group base rather than the
segment base.
NASM will allow a segment to be part of more than one group, but will generate a warning
if you do this. Variables declared in a segment which is part of more than one group will
default to being relative to the first group that was defined to contain the segment.
A group does not have to contain any segments; you can still make WRT references to a
group which does not contain the variable you are referring to. OS/2, for example, defines
the special group FLAT with no segments in it.
6.2.3 UPPERCASE: Disabling Case Sensitivity in Output
Although NASM itself is case sensitive, some OMF linkers are not; therefore it can be
useful for NASM to output single-case object files. The UPPERCASE format-specific
directive causes all segment, group and symbol names that are written to the object file to
49
be forced to upper case just before being written. Within a source file, NASM is still casesensitive; but the object file can be written entirely in upper case if desired.
UPPERCASE
is used alone on a line; it requires no parameters.
6.2.4 IMPORT: Importing DLL Symbols
The IMPORT format-specific directive defines a symbol to be imported from a DLL, for use
if you are writing a DLL's import library in NASM. You still need to declare the symbol as
EXTERN as well as using the IMPORT directive.
The IMPORT directive takes two required parameters, separated by white space, which are
(respectively) the name of the symbol you wish to import and the name of the library you
wish to import it from. For example:
import WSAStartup wsock32.dll
A third optional parameter gives the name by which the symbol is known in the library you
are importing it from, in case this is not the same as the name you wish the symbol to be
known by to your code once you have imported it. For example:
import asyncsel wsock32.dll WSAAsyncSelect
6.2.5 EXPORT: Exporting DLL Symbols
The EXPORT format-specific directive defines a global symbol to be exported as a DLL
symbol, for use if you are writing a DLL in NASM. You still need to declare the symbol as
GLOBAL as well as using the EXPORT directive.
EXPORT
takes one required parameter, which is the name of the symbol you wish to export,
as it was defined in your source file. An optional second parameter (separated by white
space from the first) gives the external name of the symbol: the name by which you wish
the symbol to be known to programs using the DLL. If this name is the same as the internal
name, you may leave the second parameter off.
Further parameters can be given to define attributes of the exported symbol. These
parameters, like the second, are separated by white space. If further parameters are given,
the external name must also be specified, even if it is the same as the internal name. The
available attributes are:

resident indicates that the exported name is to be kept resident by the system loader. This is an
optimisation for frequently used symbols imported by name.

nodata indicates that the exported symbol is a function which does not make use of any initialised
data.

parm=NNN, where NNN is an integer, sets the number of parameter words for the case in which the
symbol is a call gate between 32-bit and 16-bit segments.

An attribute which is just a number indicates that the symbol should be exported with an identifying
number (ordinal), and gives the desired number.
For example:
50
export
export
export
export
myfunc
myfunc TheRealMoreFormalLookingFunctionName
myfunc myfunc 1234 ; export by ordinal
myfunc myfunc resident parm=23 nodata
6.2.6 ..start: Defining the Program Entry Point
OMF linkers require exactly one of the object files being linked to define the program entry
point, where execution will begin when the program is run. If the object file that defines the
entry point is assembled using NASM, you specify the entry point by declaring the special
symbol ..start at the point where you wish execution to begin.
6.2.7 obj Extensions to the EXTERN Directive
If you declare an external symbol with the directive
extern foo
then references such as mov ax,foo will give you the offset of foo from its preferred
segment base (as specified in whichever module foo is actually defined in). So to access
the contents of foo you will usually need to do something like
mov ax,seg foo
mov es,ax
mov ax,[es:foo]
; get preferred segment base
; move it into ES
; and use offset `foo' from it
This is a little unwieldy, particularly if you know that an external is going to be accessible
from a given segment or group, say dgroup. So if DS already contained dgroup, you could
simply code
mov ax,[foo wrt dgroup]
However, having to type this every time you want to access foo can be a pain; so NASM
allows you to declare foo in the alternative form
extern foo:wrt dgroup
This form causes NASM to pretend that the preferred segment base of foo is in fact
dgroup; so the expression seg foo will now return dgroup, and the expression foo is
equivalent to foo wrt dgroup.
This default-WRT mechanism can be used to make externals appear to be relative to any
group or segment in your program. It can also be applied to common variables: see section
6.2.8.
6.2.8 obj Extensions to the COMMON Directive
The obj format allows common variables to be either near or far; NASM allows you to
specify which your variables should be by the use of the syntax
common nearvar 2:near
common farvar 10:far
; `nearvar' is a near common
; and `farvar' is far
Far common variables may be greater in size than 64Kb, and so the OMF specification says
that they are declared as a number of elements of a given size. So a 10-byte far common
variable could be declared as ten one-byte elements, five two-byte elements, two five-byte
elements or one ten-byte element.
51
Some OMF linkers require the element size, as well as the variable size, to match when
resolving common variables declared in more than one module. Therefore NASM must
allow you to specify the element size on your far common variables. This is done by the
following syntax:
common c_5by2 10:far 5 ; two five-byte elements
common c_2by5 10:far 2 ; five two-byte elements
If no element size is specified, the default is 1. Also, the FAR keyword is not required when
an element size is specified, since only far commons may have element sizes at all. So the
above declarations could equivalently be
common c_5by2 10:5
common c_2by5 10:2
; two five-byte elements
; five two-byte elements
In addition to these extensions, the COMMON directive in obj also supports default-WRT
specification like EXTERN does (explained in section 6.2.7). So you can also declare things
like
common foo 10:wrt dgroup
common bar 16:far 2:wrt data
common baz 24:wrt data:6
6.3 win32: Microsoft Win32 Object Files
The win32 output format generates Microsoft Win32 object files, suitable for passing to
Microsoft linkers such as Visual C++. Note that Borland Win32 compilers do not use this
format, but use obj instead (see section 6.2).
win32
provides a default output file-name extension of .obj.
Note that although Microsoft say that Win32 object files follow the COFF (Common
Object File Format) standard, the object files produced by Microsoft Win32 compilers are
not compatible with COFF linkers such as DJGPP's, and vice versa. This is due to a
difference of opinion over the precise semantics of PC-relative relocations. To produce
COFF files suitable for DJGPP, use NASM's coff output format; conversely, the coff
format does not produce object files that Win32 linkers can generate correct output from.
6.3.1 win32 Extensions to the SECTION Directive
Like the obj format, win32 allows you to specify additional information on the SECTION
directive line, to control the type and properties of sections you declare. Section types and
properties are generated automatically by NASM for the standard section names .text,
.data and .bss, but may still be overridden by these qualifiers.
The available qualifiers are:

code, or equivalently text, defines the section to be a code section. This marks the section as
readable and executable, but not writable, and also indicates to the linker that the type of the section
is code.

data and bss define the section to be a data section, analogously to code. Data sections are
marked as readable and writable, but not executable. data declares an initialised data section,
whereas bss declares an uninitialised data section.
52

info defines the section to be an informational section, which is not included in the executable file
by the linker, but may (for example) pass information to the linker. For example, declaring an infotype section called .drectve causes the linker to interpret the contents of the section as commandline options.

align=, used with a trailing number as in obj, gives the alignment requirements of the section.
The maximum you may specify is 64: the Win32 object file format contains no means to request a
greater section alignment than this. If alignment is not explicitly specified, the defaults are 16-byte
alignment for code sections, and 4-byte alignment for data (and BSS) sections. Informational
sections get a default alignment of 1 byte (no alignment), though the value does not matter.
The defaults assumed by NASM if you do not specify the above qualifiers are:
section .text code align=16
section .data data align=4
section .bss bss align=4
Any other section name is treated by default like .text.
6.4 coff: Common Object File Format
The coff output type produces COFF object files suitable for linking with the DJGPP
linker.
coff
provides a default output file-name extension of .o.
The coff format supports the same extensions to the SECTION directive as win32 does,
except that the align qualifier and the info section type are not supported.
6.5 elf: Linux ELFObject Files
The elf output format generates ELF32 (Executable and Linkable Format) object files, as
used by Linux. elf provides a default output file-name extension of .o.
6.5.1 elf Extensions to the SECTION Directive
Like the obj format, elf allows you to specify additional information on the SECTION
directive line, to control the type and properties of sections you declare. Section types and
properties are generated automatically by NASM for the standard section names .text,
.data and .bss, but may still be overridden by these qualifiers.
The available qualifiers are:

alloc defines the section to be one which is loaded into memory when the program is run.
noalloc defines it to be one which is not, such as an informational or comment section.

exec defines the section to be one which should have execute permission when the program is run.
noexec defines it as one which should not.

write defines the section to be one which should be writable when the program is run. nowrite
defines it as one which should not.
53

progbits defines the section to be one with explicit contents stored in the object file: an ordinary
code or data section, for example, nobits defines the section to be one with no explicit contents
given, such as a BSS section.

align=, used with a trailing number as in obj, gives the alignment requirements of the section.
The defaults assumed by NASM if you do not specify the above qualifiers are:
section
section
section
section
.text progbits alloc
exec nowrite
.data progbits alloc noexec
write
.bss
nobits alloc noexec
write
other progbits alloc noexec nowrite
align=16
align=4
align=4
align=1
(Any section name other than .text, .data and .bss is treated by default like other in the
above code.)
6.5.2 Position-Independent Code: elf Special Symbols and WRT
The ELF specification contains enough features to allow position-independent code (PIC)
to be written, which makes ELF shared libraries very flexible. However, it also means
NASM has to be able to generate a variety of strange relocation types in ELF object files, if
it is to be an assembler which can write PIC.
Since ELF does not support segment-base references, the WRT operator is not used for its
normal purpose; therefore NASM's elf output format makes use of WRT for a different
purpose, namely the PIC-specific relocation types.
elf
defines five special symbols which you can use as the right-hand side of the WRT
operator to obtain PIC relocation types. They are ..gotpc, ..gotoff, ..got, ..plt and
..sym. Their functions are summarised here:

Referring to the symbol marking the global offset table base using wrt ..gotpc will end up
giving the distance from the beginning of the current section to the global offset table.
(_GLOBAL_OFFSET_TABLE_ is the standard symbol name used to refer to the GOT.) So you
would then need to add $$ to the result to get the real address of the GOT.

Referring to a location in one of your own sections using wrt ..gotoff will give the distance
from the beginning of the GOT to the specified location, so that adding on the address of the GOT
would give the real address of the location you wanted.

Referring to an external or global symbol using wrt ..got causes the linker to build an entry in
the GOT containing the address of the symbol, and the reference gives the distance from the
beginning of the GOT to the entry; so you can add on the address of the GOT, load from the
resulting address, and end up with the address of the symbol.

Referring to a procedure name using wrt ..plt causes the linker to build a procedure linkage
table entry for the symbol, and the reference gives the address of the PLT entry. You can only use
this in contexts which would generate a PC-relative relocation normally (i.e. as the destination for
CALL or JMP), since ELF contains no relocation type to refer to PLT entries absolutely.

Referring to a symbol name using wrt ..sym causes NASM to write an ordinary relocation, but
instead of making the relocation relative to the start of the section and then adding on the offset to
the symbol, it will write a relocation record aimed directly at the symbol in question. The distinction
is a necessary one due to a peculiarity of the dynamic linker.
54
A fuller explanation of how to use these relocation types to write shared libraries entirely in
NASM is given in section 8.2.
6.5.3 elf Extensions to the GLOBAL Directive
ELF object files can contain more information about a global symbol than just its address:
they can contain the size of the symbol and its type as well. These are not merely debugger
conveniences, but are actually necessary when the program being written is a shared
library. NASM therefore supports some extensions to the GLOBAL directive, allowing you to
specify these features.
You can specify whether a global variable is a function or a data object by suffixing the
name with a colon and the word function or data. (object is a synonym for data.) For
example:
global hashlookup:function, hashtable:data
exports the global symbol hashlookup as a function and hashtable as a data object.
You can also specify the size of the data associated with the symbol, as a numeric
expression (which may involve labels, and even forward references) after the type specifier.
Like this:
global hashtable:data (hashtable.end - hashtable)
hashtable:
db this,that,theother
; some data here
.end:
This makes NASM automatically calculate the length of the table and place that
information into the ELF symbol table.
Declaring the type and size of global symbols is necessary when writing shared library
code. For more information, see section 8.2.4.
6.5.4 elf Extensions to the COMMON Directive
ELF also allows you to specify alignment requirements on common variables. This is done
by putting a number (which must be a power of two) after the name and size of the
common variable, separated (as usual) by a colon. For example, an array of doublewords
would benefit from 4-byte alignment:
common dwordarray 128:4
This declares the total size of the array to be 128 bytes, and requires that it be aligned on a
4-byte boundary.
6.6 aout: Linux a.out Object Files
The aout format generates a.out object files, in the form used by early Linux systems.
(These differ from other a.out object files in that the magic number in the first four bytes
of the file is different. Also, some implementations of a.out, for example NetBSD's,
support position-independent code, which Linux's implementation doesn't.)
a.out
provides a default output file-name extension of .o.
55
a.out
is a very simple object format. It supports no special directives, no special symbols,
no use of SEG or WRT, and no extensions to any standard directives. It supports only the
three standard section names .text, .data and .bss.
6.7 aoutb: NetBSD/FreeBSD/OpenBSD a.out Object Files
The aoutb format generates a.out object files, in the form used by the various free BSD
Unix clones, NetBSD, FreeBSD and OpenBSD. For simple object files, this object format
is exactly the same as aout except for the magic number in the first four bytes of the file.
However, the aoutb format supports position-independent code in the same way as the elf
format, so you can use it to write BSD shared libraries.
aoutb
provides a default output file-name extension of .o.
aoutb
supports no special directives, no special symbols, and only the three standard
section names .text, .data and .bss. However, it also supports the same use of WRT as
elf does, to provide position-independent code relocation types. See section 6.5.2 for full
documentation of this feature.
aoutb
also supports the same extensions to the GLOBAL directive as elf does: see section
6.5.3 for documentation of this.
6.8 as86: Linux as86 Object Files
The Linux 16-bit assembler as86 has its own non-standard object file format. Although its
companion linker ld86 produces something close to ordinary a.out binaries as output, the
object file format used to communicate between as86 and ld86 is not itself a.out.
NASM supports this format, just in case it is useful, as as86. as86 provides a default output
file-name extension of .o.
as86
is a very simple object format (from the NASM user's point of view). It supports no
special directives, no special symbols, no use of SEG or WRT, and no extensions to any
standard directives. It supports only the three standard section names .text, .data and
.bss.
6.9 rdf: Relocatable Dynamic Object File Format
The rdf output format produces RDOFF object files. RDOFF (Relocatable Dynamic
Object File Format) is a home-grown object-file format, designed alongside NASM itself
and reflecting in its file format the internal structure of the assembler.
RDOFF is not used by any well-known operating systems. Those writing their own
systems, however, may well wish to use RDOFF as their object format, on the grounds that
it is designed primarily for simplicity and contains very little file-header bureaucracy.
The Unix NASM archive, and the DOS archive which includes sources, both contain an
rdoff subdirectory holding a set of RDOFF utilities: an RDF linker, an RDF static-library
56
manager, an RDF file dump utility, and a program which will load and execute an RDF
executable under Linux.
rdf
supports only the standard section names .text, .data and .bss.
6.9.1 Requiring a Library: The LIBRARY Directive
RDOFF contains a mechanism for an object file to demand a given library to be linked to
the module, either at load time or run time. This is done by the LIBRARY directive, which
takes one argument which is the name of the module:
library mylib.rdl
6.10 dbg: Debugging Format
The dbg output format is not built into NASM in the default configuration. If you are
building your own NASM executable from the sources, you can define OF_DBG in
outform.h or on the compiler command line, and obtain the dbg output format.
The dbg format does not output an object file as such; instead, it outputs a text file which
contains a complete list of all the transactions between the main body of NASM and the
output-format back end module. It is primarily intended to aid people who want to write
their own output drivers, so that they can get a clearer idea of the various requests the main
program makes of the output driver, and in what order they happen.
For simple files, one can easily use the dbg format like this:
nasm -f dbg filename.asm
which will generate a diagnostic file called filename.dbg. However, this will not work
well on files which were designed for a different object format, because each object format
defines its own macros (usually user-level forms of directives), and those macros will not
be defined in the dbg format. Therefore it can be useful to run NASM twice, in order to do
the preprocessing with the native object format selected:
nasm -e -f rdf -o rdfprog.i rdfprog.asm
nasm -a -f dbg rdfprog.i
This preprocesses rdfprog.asm into rdfprog.i, keeping the rdf object format selected in
order to make sure RDF special directives are converted into primitive form correctly. Then
the preprocessed source is fed through the dbg format to generate the final diagnostic
output.
This workaround will still typically not work for programs intended for obj format,
because the obj SEGMENT and GROUP directives have side effects of defining the segment
and group names as symbols; dbg will not do this, so the program will not assemble. You
will have to work around that by defining the symbols yourself (using EXTERN, for
example) if you really need to get a dbg trace of an obj-specific source file.
dbg
accepts any section name and any directives at all, and logs them all to its output file.
57
Chapter 7: Writing 16-bit Code (DOS, Windows 3/3.1)
This chapter attempts to cover some of the common issues encountered when writing 16-bit
code to run under MS-DOS or Windows 3.x. It covers how to link programs to produce
.EXE or .COM files, how to write .SYS device drivers, and how to interface assembly
language code with 16-bit C compilers and with Borland Pascal.
7.1 Producing .EXE Files
Any large program written under DOS needs to be built as a .EXE file: only .EXE files have
the necessary internal structure required to span more than one 64K segment. Windows
programs, also, have to be built as .EXE files, since Windows does not support the .COM
format.
In general, you generate .EXE files by using the obj output format to produce one or more
.OBJ files, and then linking them together using a linker. However, NASM also supports
the direct generation of simple DOS .EXE files using the bin output format (by using DB
and DW to construct the .EXE file header), and a macro package is supplied to do this.
Thanks to Yann Guidon for contributing the code for this.
NASM may also support .EXE natively as another output format in future releases.
7.1.1 Using the obj Format To Generate .EXE Files
This section describes the usual method of generating .EXE files by linking .OBJ files
together.
Most 16-bit programming language packages come with a suitable linker; if you have none
of these, there is a free linker called VAL, available in LZH archive format from
x2ftp.oulu.fi. An LZH archiver can be found at ftp.simtel.net. There is another
`free' linker (though this one doesn't come with sources) called FREELINK, available from
www.pcorner.com. A third, djlink, written by DJ Delorie, is available at
www.delorie.com.
When linking several .OBJ files into a .EXE file, you should ensure that exactly one of them
has a start point defined (using the ..start special symbol defined by the obj format: see
section 6.2.6). If no module defines a start point, the linker will not know what value to
give the entry-point field in the output file header; if more than one defines a start point, the
linker will not know which value to use.
An example of a NASM source file which can be assembled to a .OBJ file and linked on its
own to a .EXE is given here. It demonstrates the basic principles of defining a stack,
initialising the segment registers, and declaring a start point. This file is also provided in the
test subdirectory of the NASM archives, under the name objexe.asm.
segment code
..start:
mov ax,data
mov ds,ax
mov ax,stack
58
mov ss,ax
mov sp,stacktop
This initial piece of code sets up DS to point to the data segment, and initialises SS and SP to
point to the top of the provided stack. Notice that interrupts are implicitly disabled for one
instruction after a move into SS, precisely for this situation, so that there's no chance of an
interrupt occurring between the loads of SS and SP and not having a stack to execute on.
Note also that the special symbol ..start is defined at the beginning of this code, which
means that will be the entry point into the resulting executable file.
mov dx,hello
mov ah,9
int 0x21
The above is the main program: load DS:DX with a pointer to the greeting message (hello
is implicitly relative to the segment data, which was loaded into DS in the setup code, so
the full pointer is valid), and call the DOS print-string function.
mov ax,0x4c00
int 0x21
This terminates the program using another DOS system call.
hello:
segment data
db 'hello, world', 13, 10, '$'
The data segment contains the string we want to display.
segment stack stack
resb 64
stacktop:
The above code declares a stack segment containing 64 bytes of uninitialised stack space,
and points stacktop at the top of it. The directive segment stack stack defines a
segment called stack, and also of type STACK. The latter is not necessary to the correct
running of the program, but linkers are likely to issue warnings or errors if your program
has no segment of type STACK.
The above file, when assembled into a .OBJ file, will link on its own to a valid .EXE file,
which when run will print `hello, world' and then exit.
7.1.2 Using the bin Format To Generate .EXE Files
The .EXE file format is simple enough that it's possible to build a .EXE file by writing a
pure-binary program and sticking a 32-byte header on the front. This header is simple
enough that it can be generated using DB and DW commands by NASM itself, so that you can
use the bin output format to directly generate .EXE files.
Included in the NASM archives, in the misc subdirectory, is a file exebin.mac of macros.
It defines three macros: EXE_begin, EXE_stack and EXE_end.
To produce a .EXE file using this method, you should start by using %include to load the
exebin.mac macro package into your source file. You should then issue the EXE_begin
macro call (which takes no arguments) to generate the file header data. Then write code as
normal for the bin format - you can use all three standard sections .text, .data and .bss.
59
At the end of the file you should call the EXE_end macro (again, no arguments), which
defines some symbols to mark section sizes, and these symbols are referred to in the header
code generated by EXE_begin.
In this model, the code you end up writing starts at 0x100, just like a .COM file - in fact, if
you strip off the 32-byte header from the resulting .EXE file, you will have a valid .COM
program. All the segment bases are the same, so you are limited to a 64K program, again
just like a .COM file. Note that an ORG directive is issued by the EXE_begin macro, so you
should not explicitly issue one of your own.
You can't directly refer to your segment base value, unfortunately, since this would require
a relocation in the header, and things would get a lot more complicated. So you should get
your segment base by copying it out of CS instead.
On entry to your .EXE file, SS:SP are already set up to point to the top of a 2Kb stack. You
can adjust the default stack size of 2Kb by calling the EXE_stack macro. For example, to
change the stack size of your program to 64 bytes, you would call EXE_stack 64.
A sample program which generates a .EXE file in this way is given in the test subdirectory
of the NASM archive, as binexe.asm.
7.2 Producing .COM Files
While large DOS programs must be written as .EXE files, small ones are often better
written as .COM files. .COM files are pure binary, and therefore most easily produced using
the bin output format.
7.2.1 Using the bin Format To Generate .COM Files
.COM
files expect to be loaded at offset 100h into their segment (though the segment may
change). Execution then begins at 100h, i.e. right at the start of the program. So to write a
.COM program, you would create a source file looking like
start:
org 100h
section .text
; put your code here
section .data
; put data items here
section .bss
; put uninitialised data here
The bin format puts the .text section first in the file, so you can declare data or BSS items
before beginning to write code if you want to and the code will still end up at the front of
the file where it belongs.
The BSS (uninitialised data) section does not take up space in the .COM file itself: instead,
addresses of BSS items are resolved to point at space beyond the end of the file, on the
grounds that this will be free memory when the program is run. Therefore you should not
rely on your BSS being initialised to all zeros when you run.
To assemble the above program, you should use a command line like
60
nasm myprog.asm -fbin -o myprog.com
The bin format would produce a file called myprog if no explicit output file name were
specified, so you have to override it and give the desired file name.
7.2.2 Using the obj Format To Generate .COM Files
If you are writing a .COM program as more than one module, you may wish to assemble
several .OBJ files and link them together into a .COM program. You can do this, provided
you have a linker capable of outputting .COM files directly (TLINK does this), or
alternatively a converter program such as EXE2BIN to transform the .EXE file output from
the linker into a .COM file.
If you do this, you need to take care of several things:

The first object file containing code should start its code segment with a line like RESB 100h. This
is to ensure that the code begins at offset 100h relative to the beginning of the code segment, so that
the linker or converter program does not have to adjust address references within the file when
generating the .COM file. Other assemblers use an ORG directive for this purpose, but ORG in NASM
is a format-specific directive to the bin output format, and does not mean the same thing as it does
in MASM-compatible assemblers.

You don't need to define a stack segment.

All your segments should be in the same group, so that every time your code or data references a
symbol offset, all offsets are relative to the same segment base. This is because, when a .COM file is
loaded, all the segment registers contain the same value.
7.3 Producing .SYS Files
MS-DOS device drivers - .SYS files - are pure binary files, similar to .COM files, except that
they start at origin zero rather than 100h. Therefore, if you are writing a device driver using
the bin format, you do not need the ORG directive, since the default origin for bin is zero.
Similarly, if you are using obj, you do not need the RESB 100h at the start of your code
segment.
.SYS
files start with a header structure, containing pointers to the various routines inside the
driver which do the work. This structure should be defined at the start of the code segment,
even though it is not actually code.
For more information on the format of .SYS files, and the data which has to go in the
header structure, a list of books is given in the Frequently Asked Questions list for the
newsgroup comp.os.msdos.programmer.
7.4 Interfacing to 16-bit C Programs
This section covers the basics of writing assembly routines that call, or are called from, C
programs. To do this, you would typically write an assembly module as a .OBJ file, and
link it with your C modules to produce a mixed-language program.
7.4.1 External Symbol Names
61
C compilers have the convention that the names of all global symbols (functions or data)
they define are formed by prefixing an underscore to the name as it appears in the C
program. So, for example, the function a C programmer thinks of as printf appears to an
assembly language programmer as _printf. This means that in your assembly programs,
you can define symbols without a leading underscore, and not have to worry about name
clashes with C symbols.
If you find the underscores inconvenient, you can define macros to replace the GLOBAL and
EXTERN directives as follows:
%macro cglobal 1
global _%1
%define %1 _%1
%endmacro
%macro cextern 1
extern _%1
%define %1 _%1
%endmacro
(These forms of the macros only take one argument at a time; a %rep construct could solve
this.)
If you then declare an external like this:
cextern printf
then the macro will expand it as
extern _printf
%define printf _printf
Thereafter, you can reference printf as if it was a symbol, and the preprocessor will put
the leading underscore on where necessary.
The cglobal macro works similarly. You must use cglobal before defining the symbol in
question, but you would have had to do that anyway if you used GLOBAL.
7.4.2 Memory Models
NASM contains no mechanism to support the various C memory models directly; you have
to keep track yourself of which one you are writing for. This means you have to keep track
of the following things:

In models using a single code segment (tiny, small and compact), functions are near. This means that
function pointers, when stored in data segments or pushed on the stack as function arguments, are 16
bits long and contain only an offset field (the CS register never changes its value, and always gives
the segment part of the full function address), and that functions are called using ordinary near CALL
instructions and return using RETN (which, in NASM, is synonymous with RET anyway). This
means both that you should write your own routines to return with RETN, and that you should call
external C routines with near CALL instructions.

In models using more than one code segment (medium, large and huge), functions are far. This
means that function pointers are 32 bits long (consisting of a 16-bit offset followed by a 16-bit
segment), and that functions are called using CALL FAR (or CALL seg:offset) and return
using RETF. Again, you should therefore write your own routines to return with RETF and use
CALL FAR to call external routines.
62

In models using a single data segment (tiny, small and medium), data pointers are 16 bits long,
containing only an offset field (the DS register doesn't change its value, and always gives the
segment part of the full data item address).

In models using more than one data segment (compact, large and huge), data pointers are 32 bits
long, consisting of a 16-bit offset followed by a 16-bit segment. You should still be careful not to
modify DS in your routines without restoring it afterwards, but ES is free for you to use to access the
contents of 32-bit data pointers you are passed.

The huge memory model allows single data items to exceed 64K in size. In all other memory
models, you can access the whole of a data item just by doing arithmetic on the offset field of the
pointer you are given, whether a segment field is present or not; in huge model, you have to be more
careful of your pointer arithmetic.

In most memory models, there is a default data segment, whose segment address is kept in DS
throughout the program. This data segment is typically the same segment as the stack, kept in SS, so
that functions' local variables (which are stored on the stack) and global data items can both be
accessed easily without changing DS. Particularly large data items are typically stored in other
segments. However, some memory models (though not the standard ones, usually) allow the
assumption that SS and DS hold the same value to be removed. Be careful about functions' local
variables in this latter case.
In models with a single code segment, the segment is called _TEXT, so your code segment
must also go by this name in order to be linked into the same place as the main code
segment. In models with a single data segment, or with a default data segment, it is called
_DATA.
7.4.3 Function Definitions and Function Calls
The C calling convention in 16-bit programs is as follows. In the following description, the
words caller and callee are used to denote the function doing the calling and the function
which gets called.

The caller pushes the function's parameters on the stack, one after another, in reverse order (right to
left, so that the first argument specified to the function is pushed last).

The caller then executes a CALL instruction to pass control to the callee. This CALL is either near or
far depending on the memory model.

The callee receives control, and typically (although this is not actually necessary, in functions which
do not need to access their parameters) starts by saving the value of SP in BP so as to be able to use
BP as a base pointer to find its parameters on the stack. However, the caller was probably doing this
too, so part of the calling convention states that BP must be preserved by any C function. Hence the
callee, if it is going to set up BP as a frame pointer, must push the previous value first.

The callee may then access its parameters relative to BP. The word at [BP] holds the previous value
of BP as it was pushed; the next word, at [BP+2], holds the offset part of the return address, pushed
implicitly by CALL. In a small-model (near) function, the parameters start after that, at [BP+4]; in a
large-model (far) function, the segment part of the return address lives at [BP+4], and the
parameters begin at [BP+6]. The leftmost parameter of the function, since it was pushed last, is
accessible at this offset from BP; the others follow, at successively greater offsets. Thus, in a
function such as printf which takes a variable number of parameters, the pushing of the
parameters in reverse order means that the function knows where to find its first parameter, which
tells it the number and type of the remaining ones.
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
The callee may also wish to decrease SP further, so as to allocate space on the stack for local
variables, which will then be accessible at negative offsets from BP.

The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AX
depending on the size of the value. Floating-point results are sometimes (depending on the compiler)
returned in ST0.

Once the callee has finished processing, it restores SP from BP if it had allocated local stack space,
then pops the previous value of BP, and returns via RETN or RETF depending on memory model.

When the caller regains control from the callee, the function parameters are still on the stack, so it
typically adds an immediate constant to SP to remove them (instead of executing a number of slow
POP instructions). Thus, if a function is accidentally called with the wrong number of parameters due
to a prototype mismatch, the stack will still be returned to a sensible state since the caller, which
knows how many parameters it pushed, does the removing.
It is instructive to compare this calling convention with that for Pascal programs (described
in section 7.5.1). Pascal has a simpler convention, since no functions have variable numbers
of parameters. Therefore the callee knows how many parameters it should have been
passed, and is able to deallocate them from the stack itself by passing an immediate
argument to the RET or RETF instruction, so the caller does not have to do it. Also, the
parameters are pushed in left-to-right order, not right-to-left, which means that a compiler
can give better guarantees about sequence points without performance suffering.
Thus, you would define a function in C style in the following way. The following example
is for small model:
_myfunc:
global _myfunc
push bp
mov bp,sp
sub sp,0x40
mov bx,[bp+4]
; some more code
mov sp,bp
pop bp
ret
; 64 bytes of local stack space
; first parameter to function
; undo "sub sp,0x40" above
For a large-model function, you would replace RET by RETF, and look for the first
parameter at [BP+6] instead of [BP+4]. Of course, if one of the parameters is a pointer,
then the offsets of subsequent parameters will change depending on the memory model as
well: far pointers take up four bytes on the stack when passed as a parameter, whereas near
pointers take up two.
At the other end of the process, to call a C function from your assembly code, you would do
something like this:
myint
extern _printf
; and then, further down...
push word [myint]
; one of my integer variables
push word mystring
; pointer into my data segment
call _printf
add sp,byte 4
; `byte' saves space
; then those data items...
segment _DATA
dw 1234
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mystring
db 'This number -> %d <- should be 1234',10,0
This piece of code is the small-model assembly equivalent of the C code
int myint = 1234;
printf("This number -> %d <- should be 1234\n", myint);
In large model, the function-call code might look more like this. In this example, it is
assumed that DS already holds the segment base of the segment _DATA. If not, you would
have to initialise it first.
push word [myint]
push word seg mystring ; Now push the segment, and...
push word mystring
; ... offset of "mystring"
call far _printf
add sp,byte 6
The integer value still takes up one word on the stack, since large model does not affect the
size of the int data type. The first argument (pushed last) to printf, however, is a data
pointer, and therefore has to contain a segment and offset part. The segment should be
stored second in memory, and therefore must be pushed first. (Of course, PUSH DS would
have been a shorter instruction than PUSH WORD SEG mystring, if DS was set up as the
above example assumed.) Then the actual call becomes a far call, since functions expect far
calls in large model; and SP has to be increased by 6 rather than 4 afterwards to make up for
the extra word of parameters.
7.4.4 Accessing Data Items
To get at the contents of C variables, or to declare variables which C can access, you need
only declare the names as GLOBAL or EXTERN. (Again, the names require leading
underscores, as stated in section 7.4.1.) Thus, a C variable declared as int i can be
accessed from assembler as
extern _i
mov ax,[_i]
And to declare your own integer variable which C programs can access as extern int j,
you do this (making sure you are assembling in the _DATA segment, if necessary):
_j
global _j
dw 0
To access a C array, you need to know the size of the components of the array. For
example, int variables are two bytes long, so if a C program declares an array as int
a[10], you can access a[3] by coding mov ax,[_a+6]. (The byte offset 6 is obtained by
multiplying the desired array index, 3, by the size of the array element, 2.) The sizes of the
C base types in 16-bit compilers are: 1 for char, 2 for short and int, 4 for long and
float, and 8 for double.
To access a C data structure, you need to know the offset from the base of the structure to
the field you are interested in. You can either do this by converting the C structure
definition into a NASM structure definition (using STRUC), or by calculating the one offset
and using just that.
65
To do either of these, you should read your C compiler's manual to find out how it
organises data structures. NASM gives no special alignment to structure members in its
own STRUC macro, so you have to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like
struct {
char c;
int i;
} foo;
might be four bytes long rather than three, since the int field would be aligned to a twobyte boundary. However, this sort of feature tends to be a configurable option in the C
compiler, either using command-line options or #pragma lines, so you have to find out how
your own compiler does it.
7.4.5 c16.mac: Helper Macros for the 16-bit C Interface
Included in the NASM archives, in the misc directory, is a file c16.mac of macros. It
defines three macros: proc, arg and endproc. These are intended to be used for C-style
procedure definitions, and they automate a lot of the work involved in keeping track of the
calling convention.
An example of an assembly function using the macro set is given here:
%$i
%$j
proc _nearproc
arg
arg
mov ax,[bp + %$i]
mov bx,[bp + %$j]
add ax,[bx]
endproc
This defines _nearproc to be a procedure taking two arguments, the first (i) an integer and
the second (j) a pointer to an integer. It returns i + *j.
Note that the arg macro has an EQU as the first line of its expansion, and since the label
before the macro call gets prepended to the first line of the expanded macro, the EQU works,
defining %$i to be an offset from BP. A context-local variable is used, local to the context
pushed by the proc macro and popped by the endproc macro, so that the same argument
name can be used in later procedures. Of course, you don't have to do that.
The macro set produces code for near functions (tiny, small and compact-model code) by
default. You can have it generate far functions (medium, large and huge-model code) by
means of coding %define FARCODE. This changes the kind of return instruction generated
by endproc, and also changes the starting point for the argument offsets. The macro set
contains no intrinsic dependency on whether data pointers are far or not.
arg
can take an optional parameter, giving the size of the argument. If no size is given, 2 is
assumed, since it is likely that many function parameters will be of type int.
The large-model equivalent of the above function would look like this:
%define FARCODE
proc _farproc
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%$i
%$j
arg
arg 4
mov ax,[bp + %$i]
mov bx,[bp + %$j]
mov es,[bp + %$j + 2]
add ax,[bx]
endproc
This makes use of the argument to the arg macro to define a parameter of size 4, because j
is now a far pointer. When we load from j, we must load a segment and an offset.
7.5 Interfacing to Borland Pascal Programs
Interfacing to Borland Pascal programs is similar in concept to interfacing to 16-bit C
programs. The differences are:

The leading underscore required for interfacing to C programs is not required for Pascal.

The memory model is always large: functions are far, data pointers are far, and no data item can be
more than 64K long. (Actually, some functions are near, but only those functions that are local to a
Pascal unit and never called from outside it. All assembly functions that Pascal calls, and all Pascal
functions that assembly routines are able to call, are far.) However, all static data declared in a Pascal
program goes into the default data segment, which is the one whose segment address will be in DS
when control is passed to your assembly code. The only things that do not live in the default data
segment are local variables (they live in the stack segment) and dynamically allocated variables. All
data pointers, however, are far.

The function calling convention is different - described below.

Some data types, such as strings, are stored differently.

There are restrictions on the segment names you are allowed to use - Borland Pascal will ignore code
or data declared in a segment it doesn't like the name of. The restrictions are described below.
7.5.1 The Pascal Calling Convention
The 16-bit Pascal calling convention is as follows. In the following description, the words
caller and callee are used to denote the function doing the calling and the function which
gets called.

The caller pushes the function's parameters on the stack, one after another, in normal order (left to
right, so that the first argument specified to the function is pushed first).

The caller then executes a far CALL instruction to pass control to the callee.

The callee receives control, and typically (although this is not actually necessary, in functions which
do not need to access their parameters) starts by saving the value of SP in BP so as to be able to use
BP as a base pointer to find its parameters on the stack. However, the caller was probably doing this
too, so part of the calling convention states that BP must be preserved by any function. Hence the
callee, if it is going to set up BP as a frame pointer, must push the previous value first.

The callee may then access its parameters relative to BP. The word at [BP] holds the previous value
of BP as it was pushed. The next word, at [BP+2], holds the offset part of the return address, and
the next one at [BP+4] the segment part. The parameters begin at [BP+6]. The rightmost
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parameter of the function, since it was pushed last, is accessible at this offset from BP; the others
follow, at successively greater offsets.

The callee may also wish to decrease SP further, so as to allocate space on the stack for local
variables, which will then be accessible at negative offsets from BP.

The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AX
depending on the size of the value. Floating-point results are returned in ST0. Results of type Real
(Borland's own custom floating-point data type, not handled directly by the FPU) are returned in
DX:BX:AX. To return a result of type String, the caller pushes a pointer to a temporary string
before pushing the parameters, and the callee places the returned string value at that location. The
pointer is not a parameter, and should not be removed from the stack by the RETF instruction.

Once the callee has finished processing, it restores SP from BP if it had allocated local stack space,
then pops the previous value of BP, and returns via RETF. It uses the form of RETF with an
immediate parameter, giving the number of bytes taken up by the parameters on the stack. This
causes the parameters to be removed from the stack as a side effect of the return instruction.

When the caller regains control from the callee, the function parameters have already been removed
from the stack, so it needs to do nothing further.
Thus, you would define a function in Pascal style, taking two Integer-type parameters, in
the following way:
myfunc:
global myfunc
push bp
mov bp,sp
sub sp,0x40
mov bx,[bp+8]
mov bx,[bp+6]
; some more code
mov sp,bp
pop bp
retf 4
; 64 bytes of local stack space
; first parameter to function
; second parameter to function
; undo "sub sp,0x40" above
; total size of params is 4
At the other end of the process, to call a Pascal function from your assembly code, you
would do something like this:
extern SomeFunc
; and then, further down...
push word seg mystring ; Now push the segment, and...
push word mystring
; ... offset of "mystring"
push word [myint]
; one of my variables
call far SomeFunc
This is equivalent to the Pascal code
procedure SomeFunc(String: PChar; Int: Integer);
SomeFunc(@mystring, myint);
7.5.2 Borland Pascal Segment Name Restrictions
Since Borland Pascal's internal unit file format is completely different from OBJ, it only
makes a very sketchy job of actually reading and understanding the various information
contained in a real OBJ file when it links that in. Therefore an object file intended to be
linked to a Pascal program must obey a number of restrictions:
68

Procedures and functions must be in a segment whose name is either CODE, CSEG, or something
ending in _TEXT.

Initialised data must be in a segment whose name is either CONST or something ending in _DATA.

Uninitialised data must be in a segment whose name is either DATA, DSEG, or something ending in
_BSS.

Any other segments in the object file are completely ignored. GROUP directives and segment
attributes are also ignored.
7.5.3 Using c16.mac With Pascal Programs
The c16.mac macro package, described in section 7.4.5, can also be used to simplify
writing functions to be called from Pascal programs, if you code %define PASCAL. This
definition ensures that functions are far (it implies FARCODE), and also causes procedure
return instructions to be generated with an operand.
Defining PASCAL does not change the code which calculates the argument offsets; you must
declare your function's arguments in reverse order. For example:
%define PASCAL
proc _pascalproc
%$j
arg 4
%$i
arg
mov ax,[bp + %$i]
mov bx,[bp + %$j]
mov es,[bp + %$j + 2]
add ax,[bx]
endproc
This defines the same routine, conceptually, as the example in section 7.4.5: it defines a
function taking two arguments, an integer and a pointer to an integer, which returns the sum
of the integer and the contents of the pointer. The only difference between this code and the
large-model C version is that PASCAL is defined instead of FARCODE, and that the arguments
are declared in reverse order.
Chapter 8: Writing 32-bit Code (Unix, Win32, DJGPP)
This chapter attempts to cover some of the common issues involved when writing 32-bit
code, to run under Win32 or Unix, or to be linked with C code generated by a Unix-style C
compiler such as DJGPP. It covers how to write assembly code to interface with 32-bit C
routines, and how to write position-independent code for shared libraries.
Almost all 32-bit code, and in particular all code running under Win32, DJGPP or any of
the PC Unix variants, runs in flat memory model. This means that the segment registers and
paging have already been set up to give you the same 32-bit 4Gb address space no matter
what segment you work relative to, and that you should ignore all segment registers
completely. When writing flat-model application code, you never need to use a segment
override or modify any segment register, and the code-section addresses you pass to CALL
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and JMP live in the same address space as the data-section addresses you access your
variables by and the stack-section addresses you access local variables and procedure
parameters by. Every address is 32 bits long and contains only an offset part.
8.1 Interfacing to 32-bit C Programs
A lot of the discussion in section 7.4, about interfacing to 16-bit C programs, still applies
when working in 32 bits. The absence of memory models or segmentation worries
simplifies things a lot.
8.1.1 External Symbol Names
Most 32-bit C compilers share the convention used by 16-bit compilers, that the names of
all global symbols (functions or data) they define are formed by prefixing an underscore to
the name as it appears in the C program. However, not all of them do: the ELF specification
states that C symbols do not have a leading underscore on their assembly-language names.
The older Linux a.out C compiler, all Win32 compilers, DJGPP, and NetBSD and
FreeBSD, all use the leading underscore; for these compilers, the macros cextern and
cglobal, as given in section 7.4.1, will still work. For ELF, though, the leading underscore
should not be used.
8.1.2 Function Definitions and Function Calls
The C calling conventionThe C calling convention in 32-bit programs is as follows. In the
following description, the words caller and callee are used to denote the function doing the
calling and the function which gets called.

The caller pushes the function's parameters on the stack, one after another, in reverse order (right to
left, so that the first argument specified to the function is pushed last).

The caller then executes a near CALL instruction to pass control to the callee.

The callee receives control, and typically (although this is not actually necessary, in functions which
do not need to access their parameters) starts by saving the value of ESP in EBP so as to be able to
use EBP as a base pointer to find its parameters on the stack. However, the caller was probably doing
this too, so part of the calling convention states that EBP must be preserved by any C function.
Hence the callee, if it is going to set up EBP as a frame pointer, must push the previous value first.

The callee may then access its parameters relative to EBP. The doubleword at [EBP] holds the
previous value of EBP as it was pushed; the next doubleword, at [EBP+4], holds the return address,
pushed implicitly by CALL. The parameters start after that, at [EBP+8]. The leftmost parameter of
the function, since it was pushed last, is accessible at this offset from EBP; the others follow, at
successively greater offsets. Thus, in a function such as printf which takes a variable number of
parameters, the pushing of the parameters in reverse order means that the function knows where to
find its first parameter, which tells it the number and type of the remaining ones.

The callee may also wish to decrease ESP further, so as to allocate space on the stack for local
variables, which will then be accessible at negative offsets from EBP.

The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or EAX
depending on the size of the value. Floating-point results are typically returned in ST0.
70

Once the callee has finished processing, it restores ESP from EBP if it had allocated local stack
space, then pops the previous value of EBP, and returns via RET (equivalently, RETN).

When the caller regains control from the callee, the function parameters are still on the stack, so it
typically adds an immediate constant to ESP to remove them (instead of executing a number of slow
POP instructions). Thus, if a function is accidentally called with the wrong number of parameters due
to a prototype mismatch, the stack will still be returned to a sensible state since the caller, which
knows how many parameters it pushed, does the removing.
There is an alternative calling convention used by Win32 programs for Windows API calls,
and also for functions called by the Windows API such as window procedures: they follow
what Microsoft calls the __stdcall convention. This is slightly closer to the Pascal
convention, in that the callee clears the stack by passing a parameter to the RET instruction.
However, the parameters are still pushed in right-to-left order.
Thus, you would define a function in C style in the following way:
_myfunc:
global _myfunc
push ebp
mov ebp,esp
sub esp,0x40
mov ebx,[ebp+8]
; some more code
leave
ret
; 64 bytes of local stack space
; first parameter to function
; mov esp,ebp / pop ebp
At the other end of the process, to call a C function from your assembly code, you would do
something like this:
myint
mystring
extern _printf
; and then, further down...
push dword [myint]
; one of my integer variables
push dword mystring
; pointer into my data segment
call _printf
add esp,byte 8
; `byte' saves space
; then those data items...
segment _DATA
dd 1234
db 'This number -> %d <- should be 1234',10,0
This piece of code is the assembly equivalent of the C code
int myint = 1234;
printf("This number -> %d <- should be 1234\n", myint);
8.1.3 Accessing Data Items
To get at the contents of C variables, or to declare variables which C can access, you need
only declare the names as GLOBAL or EXTERN. (Again, the names require leading
underscores, as stated in section 8.1.1.) Thus, a C variable declared as int i can be
accessed from assembler as
extern _i
mov eax,[_i]
And to declare your own integer variable which C programs can access as extern int j,
you do this (making sure you are assembling in the _DATA segment, if necessary):
71
_j
global _j
dd 0
To access a C array, you need to know the size of the components of the array. For
example, int variables are four bytes long, so if a C program declares an array as int
a[10], you can access a[3] by coding mov ax,[_a+12]. (The byte offset 12 is obtained by
multiplying the desired array index, 3, by the size of the array element, 4.) The sizes of the
C base types in 32-bit compilers are: 1 for char, 2 for short, 4 for int, long and float,
and 8 for double. Pointers, being 32-bit addresses, are also 4 bytes long.
To access a C data structure, you need to know the offset from the base of the structure to
the field you are interested in. You can either do this by converting the C structure
definition into a NASM structure definition (using STRUC), or by calculating the one offset
and using just that.
To do either of these, you should read your C compiler's manual to find out how it
organises data structures. NASM gives no special alignment to structure members in its
own STRUC macro, so you have to specify alignment yourself if the C compiler generates it.
Typically, you might find that a structure like
struct {
char c;
int i;
} foo;
might be eight bytes long rather than five, since the int field would be aligned to a fourbyte boundary. However, this sort of feature is sometimes a configurable option in the C
compiler, either using command-line options or #pragma lines, so you have to find out how
your own compiler does it.
8.1.4 c32.mac: Helper Macros for the 32-bit C Interface
Included in the NASM archives, in the misc directory, is a file c32.mac of macros. It
defines three macros: proc, arg and endproc. These are intended to be used for C-style
procedure definitions, and they automate a lot of the work involved in keeping track of the
calling convention.
An example of an assembly function using the macro set is given here:
%$i
%$j
proc _proc32
arg
arg
mov eax,[ebp + %$i]
mov ebx,[ebp + %$j]
add eax,[ebx]
endproc
This defines _proc32 to be a procedure taking two arguments, the first (i) an integer and
the second (j) a pointer to an integer. It returns i + *j.
Note that the arg macro has an EQU as the first line of its expansion, and since the label
before the macro call gets prepended to the first line of the expanded macro, the EQU works,
defining %$i to be an offset from BP. A context-local variable is used, local to the context
72
pushed by the proc macro and popped by the endproc macro, so that the same argument
name can be used in later procedures. Of course, you don't have to do that.
arg
can take an optional parameter, giving the size of the argument. If no size is given, 4 is
assumed, since it is likely that many function parameters will be of type int or pointers.
8.2 Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries
ELF replaced the older a.out object file format under Linux because it contains support for
position-independent code (PIC), which makes writing shared libraries much easier. NASM
supports the ELF position-independent code features, so you can write Linux ELF shared
libraries in NASM.
NetBSD, and its close cousins FreeBSD and OpenBSD, take a different approach by
hacking PIC support into the a.out format. NASM supports this as the aoutb output
format, so you can write BSD shared libraries in NASM too.
The operating system loads a PIC shared library by memory-mapping the library file at an
arbitrarily chosen point in the address space of the running process. The contents of the
library's code section must therefore not depend on where it is loaded in memory.
Therefore, you cannot get at your variables by writing code like this:
mov eax,[myvar]
; WRONG
Instead, the linker provides an area of memory called the global offset table, or GOT; the
GOT is situated at a constant distance from your library's code, so if you can find out where
your library is loaded (which is typically done using a CALL and POP combination), you can
obtain the address of the GOT, and you can then load the addresses of your variables out of
linker-generated entries in the GOT.
The data section of a PIC shared library does not have these restrictions: since the data
section is writable, it has to be copied into memory anyway rather than just paged in from
the library file, so as long as it's being copied it can be relocated too. So you can put
ordinary types of relocation in the data section without too much worry (but see section
8.2.4 for a caveat).
8.2.1 Obtaining the Address of the GOT
Each code module in your shared library should define the GOT as an external symbol:
extern _GLOBAL_OFFSET_TABLE_
extern __GLOBAL_OFFSET_TABLE_
; in ELF
; in BSD a.out
At the beginning of any function in your shared library which plans to access your data or
BSS sections, you must first calculate the address of the GOT. This is typically done by
writing the function in this form:
func:
push ebp
mov ebp,esp
push ebx
call .get_GOT
.get_GOT: pop ebx
add ebx,_GLOBAL_OFFSET_TABLE_+$$-.get_GOT wrt ..gotpc
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; the function body comes here
mov ebx,[ebp-4]
mov esp,ebp
pop ebp
ret
(For BSD, again, the symbol _GLOBAL_OFFSET_TABLE requires a second leading
underscore.)
The first two lines of this function are simply the standard C prologue to set up a stack
frame, and the last three lines are standard C function epilogue. The third line, and the
fourth to last line, save and restore the EBX register, because PIC shared libraries use this
register to store the address of the GOT.
The interesting bit is the CALL instruction and the following two lines. The CALL and POP
combination obtains the address of the label .get_GOT, without having to know in advance
where the program was loaded (since the CALL instruction is encoded relative to the current
position). The ADD instruction makes use of one of the special PIC relocation types:
GOTPC relocation. With the WRT ..gotpc qualifier specified, the symbol referenced (here
_GLOBAL_OFFSET_TABLE_, the special symbol assigned to the GOT) is given as an offset
from the beginning of the section. (Actually, ELF encodes it as the offset from the operand
field of the ADD instruction, but NASM simplifies this deliberately, so you do things the
same way for both ELF and BSD.) So the instruction then adds the beginning of the
section, to get the real address of the GOT, and subtracts the value of .get_GOT which it
knows is in EBX. Therefore, by the time that instruction has finished, EBX contains the
address of the GOT.
If you didn't follow that, don't worry: it's never necessary to obtain the address of the GOT
by any other means, so you can put those three instructions into a macro and safely ignore
them:
%macro get_GOT 0
call %%getgot
%%getgot: pop ebx
add ebx,_GLOBAL_OFFSET_TABLE_+$$-%%getgot wrt ..gotpc
%endmacro
8.2.2 Finding Your Local Data Items
Having got the GOT, you can then use it to obtain the addresses of your data items. Most
variables will reside in the sections you have declared; they can be accessed using the
..gotoff special WRT type. The way this works is like this:
lea eax,[ebx+myvar wrt ..gotoff]
The expression myvar wrt ..gotoff is calculated, when the shared library is linked, to be
the offset to the local variable myvar from the beginning of the GOT. Therefore, adding it
to EBX as above will place the real address of myvar in EAX.
If you declare variables as GLOBAL without specifying a size for them, they are shared
between code modules in the library, but do not get exported from the library to the
program that loaded it. They will still be in your ordinary data and BSS sections, so you can
access them in the same way as local variables, using the above ..gotoff mechanism.
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Note that due to a peculiarity of the way BSD a.out format handles this relocation type,
there must be at least one non-local symbol in the same section as the address you're trying
to access.
8.2.3 Finding External and Common Data Items
If your library needs to get at an external variable (external to the library, not just to one of
the modules within it), you must use the ..got type to get at it. The ..got type, instead of
giving you the offset from the GOT base to the variable, gives you the offset from the GOT
base to a GOT entry containing the address of the variable. The linker will set up this GOT
entry when it builds the library, and the dynamic linker will place the correct address in it at
load time. So to obtain the address of an external variable extvar in EAX, you would code
mov eax,[ebx+extvar wrt ..got]
This loads the address of extvar out of an entry in the GOT. The linker, when it builds the
shared library, collects together every relocation of type ..got, and builds the GOT so as to
ensure it has every necessary entry present.
Common variables must also be accessed in this way.
8.2.4 Exporting Symbols to the Library User
If you want to export symbols to the user of the library, you have to declare whether they
are functions or data, and if they are data, you have to give the size of the data item. This is
because the dynamic linker has to build procedure linkage table entries for any exported
functions, and also moves exported data items away from the library's data section in which
they were declared.
So to export a function to users of the library, you must use
func:
global func:function
push ebp
; etc.
; declare it as a function
And to export a data item such as an array, you would have to code
array:
.end:
global array:data array.end-array ; give the size too
resd 128
Be careful: If you export a variable to the library user, by declaring it as GLOBAL and
supplying a size, the variable will end up living in the data section of the main program,
rather than in your library's data section, where you declared it. So you will have to access
your own global variable with the ..got mechanism rather than ..gotoff, as if it were
external (which, effectively, it has become).
Equally, if you need to store the address of an exported global in one of your data sections,
you can't do it by means of the standard sort of code:
dataptr:
dd global_data_item
; WRONG
NASM will interpret this code as an ordinary relocation, in which global_data_item is
merely an offset from the beginning of the .data section (or whatever); so this reference
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will end up pointing at your data section instead of at the exported global which resides
elsewhere.
Instead of the above code, then, you must write
dataptr:
dd global_data_item wrt ..sym
which makes use of the special WRT type ..sym to instruct NASM to search the symbol
table for a particular symbol at that address, rather than just relocating by section base.
Either method will work for functions: referring to one of your functions by means of
funcptr:
dd my_function
will give the user the address of the code you wrote, whereas
funcptr:
dd my_function wrt ..sym
will give the address of the procedure linkage table for the function, which is where the
calling program will believe the function lives. Either address is a valid way to call the
function.
8.2.5 Calling Procedures Outside the Library
Calling procedures outside your shared library has to be done by means of a procedure
linkage table, or PLT. The PLT is placed at a known offset from where the library is
loaded, so the library code can make calls to the PLT in a position-independent way.
Within the PLT there is code to jump to offsets contained in the GOT, so function calls to
other shared libraries or to routines in the main program can be transparently passed off to
their real destinations.
To call an external routine, you must use another special PIC relocation type, WRT ..plt.
This is much easier than the GOT-based ones: you simply replace calls such as CALL
printf with the PLT-relative version CALL printf WRT ..plt.
8.2.6 Generating the Library File
Having written some code modules and assembled them to .o files, you then generate your
shared library with a command such as
ld -shared -o library.so module1.o module2.o
ld -Bshareable -o library.so module1.o module2.o
# for ELF
# for BSD
For ELF, if your shared library is going to reside in system directories such as /usr/lib or
/lib, it is usually worth using the -soname flag to the linker, to store the final library file
name, with a version number, into the library:
ld -shared -soname library.so.1 -o library.so.1.2 *.o
You would then copy library.so.1.2 into the library directory, and create library.so.1
as a symbolic link to it.
Chapter 9: Mixing 16 and 32 Bit Code
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This chapter tries to cover some of the issues, largely related to unusual forms of addressing
and jump instructions, encountered when writing operating system code such as protectedmode initialisation routines, which require code that operates in mixed segment sizes, such
as code in a 16-bit segment trying to modify data in a 32-bit one, or jumps between
different-size segments.
9.1 Mixed-Size Jumps
The most common form of mixed-size instruction is the one used when writing a 32-bit OS:
having done your setup in 16-bit mode, such as loading the kernel, you then have to boot it
by switching into protected mode and jumping to the 32-bit kernel start address. In a fully
32-bit OS, this tends to be the only mixed-size instruction you need, since everything before
it can be done in pure 16-bit code, and everything after it can be pure 32-bit.
This jump must specify a 48-bit far address, since the target segment is a 32-bit one.
However, it must be assembled in a 16-bit segment, so just coding, for example,
jmp 0x1234:0x56789ABC
; wrong!
will not work, since the offset part of the address will be truncated to 0x9ABC and the jump
will be an ordinary 16-bit far one.
The Linux kernel setup code gets round the inability of as86 to generate the required
instruction by coding it manually, using DB instructions. NASM can go one better than that,
by actually generating the right instruction itself. Here's how to do it right:
jmp dword 0x1234:0x56789ABC
; right
The DWORD prefix (strictly speaking, it should come after the colon, since it is declaring the
offset field to be a doubleword; but NASM will accept either form, since both are
unambiguous) forces the offset part to be treated as far, in the assumption that you are
deliberately writing a jump from a 16-bit segment to a 32-bit one.
You can do the reverse operation, jumping from a 32-bit segment to a 16-bit one, by means
of the WORD prefix:
jmp word 0x8765:0x4321 ; 32 to 16 bit
If the WORD prefix is specified in 16-bit mode, or the DWORD prefix in 32-bit mode, they will
be ignored, since each is explicitly forcing NASM into a mode it was in anyway.
9.2 Addressing Between Different-Size Segments
If your OS is mixed 16 and 32-bit, or if you are writing a DOS extender, you are likely to
have to deal with some 16-bit segments and some 32-bit ones. At some point, you will
probably end up writing code in a 16-bit segment which has to access data in a 32-bit
segment, or vice versa.
If the data you are trying to access in a 32-bit segment lies within the first 64K of the
segment, you may be able to get away with using an ordinary 16-bit addressing operation
for the purpose; but sooner or later, you will want to do 32-bit addressing from 16-bit
mode.
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The easiest way to do this is to make sure you use a register for the address, since any
effective address containing a 32-bit register is forced to be a 32-bit address. So you can do
mov eax,offset_into_32_bit_segment_specified_by_fs
mov dword [fs:eax],0x11223344
This is fine, but slightly cumbersome (since it wastes an instruction and a register) if you
already know the precise offset you are aiming at. The x86 architecture does allow 32-bit
effective addresses to specify nothing but a 4-byte offset, so why shouldn't NASM be able
to generate the best instruction for the purpose?
It can. As in section 9.1, you need only prefix the address with the DWORD keyword, and it
will be forced to be a 32-bit address:
mov dword [fs:dword my_offset],0x11223344
Also as in section 9.1, NASM is not fussy about whether the DWORD prefix comes before or
after the segment override, so arguably a nicer-looking way to code the above instruction is
mov dword [dword fs:my_offset],0x11223344
Don't confuse the DWORD prefix outside the square brackets, which controls the size of the
data stored at the address, with the one inside the square brackets which controls the
length of the address itself. The two can quite easily be different:
mov word [dword 0x12345678],0x9ABC
This moves 16 bits of data to an address specified by a 32-bit offset.
You can also specify WORD or DWORD prefixes along with the FAR prefix to indirect far jumps
or calls. For example:
call dword far [fs:word 0x4321]
This instruction contains an address specified by a 16-bit offset; it loads a 48-bit far pointer
from that (16-bit segment and 32-bit offset), and calls that address.
9.3 Other Mixed-Size Instructions
The other way you might want to access data might be using the string instructions (LODSx,
STOSx and so on) or the XLATB instruction. These instructions, since they take no
parameters, might seem to have no easy way to make them perform 32-bit addressing when
assembled in a 16-bit segment.
This is the purpose of NASM's a16 and a32 prefixes. If you are coding LODSB in a 16-bit
segment but it is supposed to be accessing a string in a 32-bit segment, you should load the
desired address into ESI and then code
a32 lodsb
The prefix forces the addressing size to 32 bits, meaning that LODSB loads from [DS:ESI]
instead of [DS:SI]. To access a string in a 16-bit segment when coding in a 32-bit one, the
corresponding a16 prefix can be used.
The a16 and a32 prefixes can be applied to any instruction in NASM's instruction table, but
most of them can generate all the useful forms without them. The prefixes are necessary
only for instructions with implicit addressing: CMPSx (section A.19), SCASx (section A.149),
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LODSx
OUTSx
(section A.98), STOSx (section A.157), MOVSx (section A.105), INSx (section A.80),
(section A.112), and XLATB (section A.169). Also, the various push and pop
instructions (PUSHA and POPF as well as the more usual PUSH and POP) can accept a16 or
a32 prefixes to force a particular one of SP or ESP to be used as a stack pointer, in case the
stack segment in use is a different size from the code segment.
PUSH
and POP, when applied to segment registers in 32-bit mode, also have the slightly odd
behaviour that they push and pop 4 bytes at a time, of which the top two are ignored and the
bottom two give the value of the segment register being manipulated. To force the 16-bit
behaviour of segment-register push and pop instructions, you can use the operand-size
prefix o16:
o16 push ss
o16 push ds
This code saves a doubleword of stack space by fitting two segment registers into the space
which would normally be consumed by pushing one.
(You can also use the o32 prefix to force the 32-bit behaviour when in 16-bit mode, but this
seems less useful.)
Chapter 10: Troubleshooting
This chapter describes some of the common problems that users have been known to
encounter with NASM, and answers them. It also gives instructions for reporting bugs in
NASM if you find a difficulty that isn't listed here.
10.1 Common Problems
10.1.1 NASM Generates Inefficient Code
I get a lot of `bug' reports about NASM generating inefficient, or even `wrong', code on
instructions such as ADD ESP,8. This is a deliberate design feature, connected to
predictability of output: NASM, on seeing ADD ESP,8, will generate the form of the
instruction which leaves room for a 32-bit offset. You need to code ADD ESP,BYTE 8 if you
want the space-efficient form of the instruction. This isn't a bug: at worst it's a misfeature,
and that's a matter of opinion only.
10.1.2 My Jumps are Out of Range
Similarly, people complain that when they issue conditional jumps (which are SHORT by
default) that try to jump too far, NASM reports `short jump out of range' instead of making
the jumps longer.
This, again, is partly a predictability issue, but in fact has a more practical reason as well.
NASM has no means of being told what type of processor the code it is generating will be
run on; so it cannot decide for itself that it should generate Jcc NEAR type instructions,
because it doesn't know that it's working for a 386 or above. Alternatively, it could replace
the out-of-range short JNE instruction with a very short JE instruction that jumps over a JMP
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NEAR;
this is a sensible solution for processors below a 386, but hardly efficient on
processors which have good branch prediction and could have used JNE NEAR instead. So,
once again, it's up to the user, not the assembler, to decide what instructions should be
generated.
10.1.3 ORG Doesn't Work
People writing boot sector programs in the bin format often complain that ORG doesn't
work the way they'd like: in order to place the 0xAA55 signature word at the end of a 512byte boot sector, people who are used to MASM tend to code
ORG 0
; some boot sector code
ORG 510
DW 0xAA55
This is not the intended use of the ORG directive in NASM, and will not work. The correct
way to solve this problem in NASM is to use the TIMES directive, like this:
ORG 0
; some boot sector code
TIMES 510-($-$$) DB 0
DW 0xAA55
The TIMES directive will insert exactly enough zero bytes into the output to move the
assembly point up to 510. This method also has the advantage that if you accidentally fill
your boot sector too full, NASM will catch the problem at assembly time and report it, so
you won't end up with a boot sector that you have to disassemble to find out what's wrong
with it.
10.1.4 TIMES Doesn't Work
The other common problem with the above code is people who write the TIMES line as
TIMES 510-$ DB 0
by reasoning that $ should be a pure number, just like 510, so the difference between them
is also a pure number and can happily be fed to TIMES.
NASM is a modular assembler: the various component parts are designed to be easily
separable for re-use, so they don't exchange information unnecessarily. In consequence, the
bin output format, even though it has been told by the ORG directive that the .text section
should start at 0, does not pass that information back to the expression evaluator. So from
the evaluator's point of view, $ isn't a pure number: it's an offset from a section base.
Therefore the difference between $ and 510 is also not a pure number, but involves a
section base. Values involving section bases cannot be passed as arguments to TIMES.
The solution, as in the previous section, is to code the TIMES line in the form
TIMES 510-($-$$) DB 0
in which $ and $$ are offsets from the same section base, and so their difference is a pure
number. This will solve the problem and generate sensible code.
10.2 Bugs
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We have never yet released a version of NASM with any known bugs. That doesn't usually
stop there being plenty we didn't know about, though. Any that you find should be reported
to hpa@zytor.com.
Please read section 2.2 first, and don't report the bug if it's listed in there as a deliberate
feature. (If you think the feature is badly thought out, feel free to send us reasons why you
think it should be changed, but don't just send us mail saying `This is a bug' if the
documentation says we did it on purpose.) Then read section 10.1, and don't bother
reporting the bug if it's listed there.
If you do report a bug, please give us all of the following information:

What operating system you're running NASM under. DOS, Linux, NetBSD, Win16, Win32, VMS
(I'd be impressed), whatever.

If you're running NASM under DOS or Win32, tell us whether you've compiled your own executable
from the DOS source archive, or whether you were using the standard distribution binaries out of the
archive. If you were using a locally built executable, try to reproduce the problem using one of the
standard binaries, as this will make it easier for us to reproduce your problem prior to fixing it.

Which version of NASM you're using, and exactly how you invoked it. Give us the precise
command line, and the contents of the NASM environment variable if any.

Which versions of any supplementary programs you're using, and how you invoked them. If the
problem only becomes visible at link time, tell us what linker you're using, what version of it you've
got, and the exact linker command line. If the problem involves linking against object files generated
by a compiler, tell us what compiler, what version, and what command line or options you used. (If
you're compiling in an IDE, please try to reproduce the problem with the command-line version of
the compiler.)

If at all possible, send us a NASM source file which exhibits the problem. If this causes copyright
problems (e.g. you can only reproduce the bug in restricted-distribution code) then bear in mind the
following two points: firstly, we guarantee that any source code sent to us for the purposes of
debugging NASM will be used only for the purposes of debugging NASM, and that we will delete
all our copies of it as soon as we have found and fixed the bug or bugs in question; and secondly, we
would prefer not to be mailed large chunks of code anyway. The smaller the file, the better. A threeline sample file that does nothing useful except demonstrate the problem is much easier to work with
than a fully fledged ten-thousand-line program. (Of course, some errors do only crop up in large
files, so this may not be possible.)

A description of what the problem actually is. `It doesn't work' is not a helpful description! Please
describe exactly what is happening that shouldn't be, or what isn't happening that should. Examples
might be: `NASM generates an error message saying Line 3 for an error that's actually on Line 5';
`NASM generates an error message that I believe it shouldn't be generating at all'; `NASM fails to
generate an error message that I believe it should be generating'; `the object file produced from this
source code crashes my linker'; `the ninth byte of the output file is 66 and I think it should be 77
instead'.

If you believe the output file from NASM to be faulty, send it to us. That allows us to determine
whether our own copy of NASM generates the same file, or whether the problem is related to
portability issues between our development platforms and yours. We can handle binary files mailed
to us as MIME attachments, uuencoded, and even BinHex. Alternatively, we may be able to provide
an FTP site you can upload the suspect files to; but mailing them is easier for us.
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
Any other information or data files that might be helpful. If, for example, the problem involves
NASM failing to generate an object file while TASM can generate an equivalent file without trouble,
then send us both object files, so we can see what TASM is doing differently from us.
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